Prepare Rails apps for integration specs

Test files are copied from the old Roadie codebase, and no real changes
have taken place yet. The contents of these files will be adjusted
shortly.
No need to check for 1.9; 1.8 is unsupported
Rename callbacks to match roadie naming
Add asset_providers option to Options
Start working on basic integration

There's a lot of indirection here. I'm not really happy about it yet,
but I think it will pay back when options are getting fetched from all
around and merged at some point.

If it doesn't pay back, one could always just remove some of the
intermediary steps.
Fix error in Guardfile
Oops. Correct the MailInliner construction inside Mailer
Remove unwanted examples from integration tests; add custom matchers
Use latest master of roadie during development
Add railtie with roadie configuration for rails apps

This made me realize that Options need to be mutable. Added a TODO to
restructure the examples for it.
Ignore .ruby-version so I can have a local setting
Improve Guard settings
Implement Options#combine[!], #merge[!]

This makes it easier to override specific options on-demand
Improve Railtie and write tests for it

I have to modify private instance variables in order to properly test
it. That's just the way Railties work; it's global state that is loaded
once. Hard to test that properly. :-(
Rework embedded rails apps; enable a few integration tests
Rename README to SPEC; add placeholder README

I hope to help people who mistakenly try to use this gem before it's
even done. Closes #2.
Document the new #combine and #merge methods
Improve setup script

It can now be run from another directory and produces prettier output.