Added a new function islandora_user_access() to simplify access callbacks.

This function will check the given object/datastream against the given
permissions and required content models using the given user. If no user is
given the user identified by the GET token will be used, if no token is present
the global user will be used.
Standards Compliance
Added tests for islandora_user_access().

Caught a spelling mistake as part of the testing writing.
Merge pull request #291 from nigelgbanks/7.x-access-cleanup

Added a new function islandora_user_access() to simplify access callbacks