Improved CharacterAnalyzer

* space + tab is called "White" in the 1.2 spec
* Used the more precise name IsAlphaNumericDashOrUnderscore
* Merged methods by using default parameter values
Improved EventReader

* Changed some naming
* Made use of ParsingEvent.NestingIncrease
Improved SimpleKey

* Removed docs since it is non-public
* Now sets SimpleKey.Mark to Mark.Empty in default ctor
Improved StringLookAheadBuffer

* Minor code changes
Improved TagDirectiveCollection and Version

* Minor code changes