Finished Constraints class (main constraints are implemented):

CodeSharper.Core/Common/ConstraintChecking:
  - Renamed ConstraintUtils to ConstraintChecking
CodeSharper.Core/Common/ConstraintChecking/Constraints:
  - Added new constraints to the class: NotNull, NotEmpty, NotBlank
CodeSharper.Core/Common/ConstraintChecking/NotEmptyConstraint:
  - Added new constraint class: NotEmptyConstraint
Started to implement text commands to CodeSharper.
CodeSharper/CodeSharper.Core/Texts:
  - Added TextNode class to Texts namespace, which represents nodes of text (TextNode.cs)
  - Added test methods to TextNode class (TextNodeTestFixture.cs)
  - Created a TextNodeHelper class for extensions methods (TextNodeHelper.cs)