Added feature to refresh lock screen on OSX 10.10

Hi @chrishunt I had just stumbled upon your podcast (via Smashing Robots) and heard about this little gem which works like a charm.

#### What's this PR do?

On Mac 10.10 (Yosemite) there is a blurred version of the wallpaper set for the login/lock screen but these were not getting updated through the `desktop set <filename>` command. I have added a step that removes this cache file so the next time the lock/login screen are visited the `png` will be regenerated from the new wallpaper.

Please feel free to double check but this should not cause any backwards compatibility issues as I check for the cache files existence before removing it.

#### Where should the reviewer start?

Changes only in ' lib/desktop/osx/osx.rb` and corresponding test file

#### How should this be manually tested?

Find yourself a yosemite machine `desktop set <something-great>` then lock or logout the machine.

#### Screenshots (if appropriate)

Screen in question
![Uploading image.png . . .]()

#### Questions:
- Does this add new dependencies into the application? **Nope**