Added support to limit synching to specified directories

See #63 

* Adds `rsync_limited` var
* Adds `:limited` task to both `:up` and `:down`

NOTE: Rather than change the functionality of the current `:files` task I thought it better to add an `:limited` task to synch the directories specified (with `/uploads` as the default). This is really useful when converting really old sites with lots of cruft over to Genesis.

To test on a Genesis project run
```
bower install git@github.com:genesis/wordpress.git#63-rsync-uploads-dir
```
and select the branch when prompted.