Support building a deployable and self-executable WAR file of Squash

Work:

- [x] Create (optionally) self-executable WAR file
- [x] Bundle assets into WAR file (relies on #53)
- [x] Make repo clone path (currently hardcoded to `tmp/repos`) configurable since this path is inside the WAR and doesn't accommodate multiple instances running simultaneously (awaiting merge in #61).
- [x] Currently only `rake` sub-task works, get all bundler bin stubs executable from the WAR (see jruby/warbler#136 and jruby/warbler#141)
- [ ] `warbler` gem should only be in `:development` group only. See comment in `Gemfile` for why it currently is not. Tracked as jruby/warbler#142.
- [x] Allow for configuration to be provided post-compilation so that production config details are not needed on developer/deployer machines or CI. This could be done a few ways (looking for input):
  - Adding an initializer that provides Configoro with an additional directory of configs to load (though this doesn't account for `config/database.yml`)
  - Adjusting Configoro's YAML loading to do ERB-interpolation (like Rails does) to provide config with Java properties or `ENV` vars
  - Teach Configoro how to be overridden by Java properties or `ENV`
  - Put an interface between config setting and config usage and have different adapters (`ENV` adapter, for example — useful for Heroku deployers)
  - Whatever comes out of #36
  - Something much simpler that I am yet to think of, since known of the above solutions are silver bullets or clearly improvements.