Merge pull request #51 from ddobrev/master

Generated abstract methods.
Check for typedefs in HasDeclarations.
Improved the CLI include collector to keep track of originating translation units.
Cleaned up usings.
Fixed VTable walking to actually walk through all the tables.
Emit a debug info instead of an error for "invalid" operator overloads.
Remove the type map finding prints to not clutter the output.
Fixed wrapping of null pointers to actually return null CLR references.
Added new pass CheckVirtualOverrideReturnCovariance that checks if a pair of return types in overriden methods are covariant according to the C++ standard.
Remove unused includes.
Cleaned up Parser.h.
Fixed parsing of C++ methods constness.
Added a new pass that checks for ambiguous overloads.
Added default constructors triviality information to the AST.
Fixed support for classes with trivial default constructors.
Updated the parser to work with calling convention changes introduced in the latest Clang revisions (r189412).
Added a new option GenerateInternalImports that outputs "__Internal" as the library name for DllImports. Fixes #54.

For more details about what it does:  http://www.mono-project.com/Embedding_Mono#Exposing_C_code_to_the_CIL_universe).
The check for code gen info is now a bit more robust.
Renamed the CLI language generator kind to be more uniform.
Fixed generation of native object instantiation to use the original names.