Add option to manage the gitolite UMASK

The default UMASK for gitolite is '0077', which is too restrictive for
third-party applications (e.g. web-based git browsers).  This adds a
'umask' parameter that defaults to '0077'.  Uses file_line to manage the
.gitolite.rc config option for UMASK.
Merge pull request #3 from joshbeard/manage_umask

Add option to manage the gitolite UMASK