Added ability for extensions to be loaded externally.

Also fixed some CSS things.
Merge pull request #361 from mharis/master

Output support added for dimensions field
Whoops.
Pass output CSS as a value to the compiler hook and allow the output css to not be added to the header.
Fix for args
PhpDoc fix: $parent
Proper inheritance on all field types.
Another small fix
Fix for issues I created with most recent post.
Fix a small change.
Fix for issue #365
Fix for compiler css output.
$units in output method was using the wrong array