(#2927) Support symbolic file modes.

This adds a new feature, support for symbolic file modes, to Puppet. In
addition to being able to specify the octal mode, you can now use the same
symbolic mode style that chmod supports:

file { "/example": mode => "u=rw,go=r" }

This also supports relative file modes:

file { "/relative": mode = "u+w,go-wx" }

Support is based on the common GNU and BSD symbolic modes of operation; you
specify a comma separated list of actions to take in each you can sit:

The user (u), group (g), other (o), or all (a) of the permission map.

You can modify the ability to read (r), write (w), execute / search (x) on a
file or directory.

You can also modify the sticky bit (t), or the setuid and setgid bits (s).

Finally, you can set conditional execute permissions (X), which will result in
the file having the execute bit if the target is a directory, or if the target
had *any* execute bit set. (eg: g+X will set x if the original was u=x,g=.)

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>