* Repositories, style values and style keys implemented

* Styles handling re-implemented for styles reusing

* Test fixed

* Reference files updated (minor changes in XML: items reordered, default style elements skipped)

* Performance optimization by reducing number of XLStyle creations

* Performance optimization. Benchmark added

* Fix conditional format initialization

* Non-initialized cell excluded from children of XLColumn and XLRow. Reference files updated (cells with default styles excluded)

* Eliminate redundant XLStyle instances creation (performance impact)

* Exclude empty font, border and fill sections in differential styles

* Missing documentation added. Auto format

* Auto format

* Empty fills and borders considered equal no matter what colors they are.

* Merge conflicts solved. Reference files updated (redundant diff. styles removed)

* Undo some whitespace changes

* Implement ToString() for style keys. Make constructors of abstract classes protected. Remove a style benchmark

* Fix ArgumentNullException usage.

* Make new Value and Key classes and structs internal (#2)

* Make new Value and Key classes and structs internal

* Code style enhancements

* Equality operators for XLStyleValue implemented.

* XLRepositoryBase.ContainsKey method improved. Documentation added.