don't require checksum on Firefox nightly

otherwise this Cask will break nightly
Merge pull request #252 from rolandwalker/firefox_nightly_no_check

don't require checksum on Firefox nightly
Update Contribution guide with correct repo path
Update link to homebrew-cask contribution guide
Add Adium Beta Cask
Merge pull request #254 from chrischernoff/adium-beta

Add Adium Beta Cask
Merge pull request #253 from chrischernoff/contribution-guidelines

Update paths in Contribution guidelines