Make repository clone directory configurable.

Configuration is stored in `config/environments/common/repositories.yml`
and supports both absolute and relative (to project root) paths.
Remove Square-specific config file
Abstract background job strategy from implementation; allow possibility of other types of backgrounding
Support for usernames with periods in the URL. Fixes #43
Add warbler to JRuby gems
JRuby war building support
Add script directory to warbler bundle

Thanks to jruby/warbler#141, this will allow us to do the following in
order to start a console, run a rake task, etc. directly from the WAR
file:

    java -jar squash.war -S script/rails console
Use my fork of warbler until jruby/warbler#141 is merged