(Snippet Insertion) Checkpoint
(Documentation) Added cross references to a couple SageInterface functions
(Snippet Insertion) Finshed this feature

* More documentation in Snippets.h

* Include directives are inserted when necessary

* Injection can be done two ways: by copying the snippet body
  or by copying each individual statement without the snippet
  scope.

* Local variables can be inserted at the beginning or end of
  the list of local variables at the insertion point.

* Recursive insertion can be enabled or disabled per snippet.

* Option to copy all snippet function definitions into the
  global scope of the insertion point's file.
(Snippet Insertion) Added more test cases
(Unparser) Disabled an assertion, but not the warning message

This assertion prevented a deep-copy of a SgFunctionDeclaration with
a defintion from being inserted into the AST for a different source
file.