improvements to RequireFile: also check that file is writable. New decorator module in extern to preserve docs and signature
check if curr_dir is ROOT.TROOT
put decorators in rootpy.decorators
file_check -> method_file_check file_cd -> method_file_cd and add file_check for any callable
decorators: comments and minor fix
add unit tests for decorators
YAGNI: remove file_check decorator
Merge pull request #200 from ndawe/master

MRG: refactor RequireFile code and add rootpy.decorators