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