Add mode parameter to FileUtils.mkdir_p

FileUtils.mkdir_p accepts now the mode parameter and sets this
for all parent directories.
Fix rm_rf when deleted dirs does not exist

FileUtils.rm_rf is implemented as an alias to FileUtils.rm.
This is wrong because then an exception is raised when any of the
deleted directories doesn't exist.
Merge pull request #254 from Werni/add_mode_parameter_to_mkdir_p

Add mode parameter to mkdir_p