Improved the usage of the Mark class

* Introduced copy ctor to Cursor.
* Had SimpleKey be constructed using a Cursor instead of a Mark.cs
* SimpleKey will itself give answers about its text positioning.
* SimpleKey.Mark now constructs a new Mark from its Cursor.
* Removed awkward public constant Initial{Line/Column}Number.
* Moved and changed the index offset test into more of a unit test
* Signed YamlDotNet.Test so that YamlDotNet could make its internals available.
Merge pull request #78 from rogernorling/mark-cursor-1

Made Mark exclusive to the external API, Cursor exclusively internal