Add standard xref checks and improved suppression

* Allow the following predefined analyses :
   undefined_function_calls, undefined_functions, locals_not_used,
   exports_not_used, deprecated_function_calls, deprecated_functions

* Trap some possible errors in case module information is not
    available

* ignore_xref works on all checks and can take {M,F,A} tuples.
  (Automatic behaviour export suppression still only works on
  exports_not_used)
Merge branch 'xref_20130130' of git://github.com/spilgames/rebar into spg-xref

Conflicts:
	src/rebar_xref.erl