Added new constraints to Constraints Framework (i should refactor to a new library).

CodeSharper.Core/Common/ConstraintChecking/Constraints:
  - Added a IsLesserThan and IsGreaterThan constraints to Constraints static class
Fixed and moved OffsetBy method to TextRange class.
CodeSharper.Core/Texts/TextDocument:
  - Fixed some bugs in UpdateText method
CodeSharper.Core/Texts/TextRange:
  - Moved OffsetBy method to this class
CodeSharper.Core/Texts/TextRangeHelper:
  - Removed this helper class