Multi-targeting: net40, net461, netstandard2.0 (#315) ...

* Convert to Visual Studio 2017 format.

* Force VS2017 appveyor image

* Add MyGet source before CI build

* Update artifacts

* Implement .NET standard as far as possible.
System.Drawing.Bitmap is still missing - hence the XLPicture API is incomplete

* Change to VS 2017 Preview image to support .NET Standard 2

* Add APPVEYOR environment variable support

* Improve color support and more

* Add a basic test for OpenXml - which currently breaks in .NET Standard 2.0

* Use OpenXML 2.7.2 for all targets

* Replace net452 with net40 build

* Try netstandard where possible

* Update NuGet on appveyor before building

* Implement MetadataExtractor to get image dimensions and format

* When setting culture, do it for every time a test is run.

* Fix dependency issue

* fix support for TIFF files

* Default the image position to top left if position hasn't been set.

* Test all targets

* Update test framework

* Add .editorconfig files back in projects

* Disable OpenXML test on AppVeyor until bug is fixed

* Better approximation of font sizes on netstandard2

* Workaround for OpenXML bug as per https://github.com/OfficeDev/Open-XML-SDK/issues/235

* Fix for DeflateStream not being seekable. Copy to MemoryStream

* Exclude Excel temporary files when building

* Use CoreCompat for measuring fonts on netstandard - remove lots of conditional code

* Reindent file

* Upgrade NUnit

* Add support for .NET 3.5

* Fix for NET35

* Remove explicit test targets. Should work on appveyor now.

* Add Microsoft.CSharp where necessary.
Remove net40 from tests, for now.

* Skip OpenXML document properties test

* Use System.Drawing.Common instead of MetadataExtractor and CoreCompat

* Sign projects

* Specify test assemblies in appveyor.yml

* Comment out platform in order to get CI to correctly detect test assembly path. as per https://github.com/appveyor/ci/issues/1746\#issuecomment-339187046

* Remove .NET3.5 build from artifacts

* Add net40 to tests

* Add reference to Microsoft.CSharp for net40

* Switch net461 to net46
Remove NU1605 warnings
Clean up dependencies

* Generate documentation file

* Ignore warning CS1591: Missing XML comment for publicly visible type or member

* Reenable some System.Drawing features for netstandard2

* Upgrade FastMember.Signed to v1.3.0

* Upgrade NUnit to 3.10

* Upgrade test framework

* Remove framework distinction for netcore

* Collapse empty XML elements for comparison - netcore and netframework versions of OpenXML save the internals differently.

* Copy text.xlsx to output directory

* Enable test for netstandard too.

* Refactor TestHelper.StripColumnWidths

* Align conditional compiler constants for netcore/netstandard to _NETSTANDARD2_0_

* Change documentation file declaration to enable build on Linux

Add unit test for conditional formatting order

Add unit test ...

Added TestHelper.CreateAndCompare for similar test as to Examples, but outside of Examples directory.
Ignore XML elements based on Uri and Regex, e.g. /docProps/core.xml 's <dcterms:> elements