NEW update to upstream rebar

This commit was done by:

$ git checkout improvements
$ git describe --tags
spil-erlang-rebar-0.6-improvements-1-gcb0dafd
$ git diff shared_deps_2013_02_24 | patch -R -p1
$ git commit -m NEW update to upstream rebar ...
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 pull request #7 from Motiejus/improvements

Improvements