[libvoikko] VoikkoSpellService in Mac Homebrew Cask

[Marko Wallin]

To simplify the installation of VoikkoSpellService on OS X I added it to 
Homebrew Cask (http://caskroom.io/). It's similar to Homebrew but for 
applications.

VoikkoSpellService can be installed with command "brew cask install 
voikkospellservice" and it links it to ~/Library/Services. The spelling 
service starts then automatically when needed.

Also the latest version (1.0.5b1) is signed with my Apple Developer ID 
which helps with Gatekeeper allowing by default apps downloaded from 
"Mac App Store and identified developers". So, users shouldn't need to 
ctrl-click the service anymore.

// Marko