Started to refactor TextSpan class

CodeSharper/CodeSharper.Core/Texts/TextSpanHelper:
  - Added TextSpanHelper class to project
  - Added OffsetBy extension method to TextSpanHelper class

CodeSharper/CodeSharper.Tests.Core/Texts/TextSpanTestFixture:
  - Added new test cases to TextSpanTestFixture class

CodeSharper/CodeSharper.Core/Common/ThrowHelper:
  - Added InvalidOperation method to ThrowHelper static class