`appraisal generate` is now working
Add default task, so `bin/appraisal` => `generate`
Speed up some test by build gem only once
Fix cucumber test failure
Adds `appraisal install`
Do less RSpec overriding
Auto relative-ize generated gemfiles

Fixes #49 and closes #50
Make `appraisal` runs install command instead
Make Thor exit with nonzero status code on failure
Add `appraisal clean` command
Add `appraisal APPRAISAL_NAME COMMAND` command
Add `appraisal COMMAND` to run the command`
Merge `APPRAISAL [APPRAISAL_NAME] COMMAND`

This prevents the need to have a description for each appraisal
subcommand as we'll add "Usage" section on top of the help command.
Add useful help message to `appraisal help`
List available appraisals when you run `help`
Add `appraisal update`

Fixes #60
Add --jobs flag (-j) to set the jobs size

Also, remove automatic CPU cores detection and let users specify the
number of jobs themselves.

Closes #62
Cleanup command calls
Remove GOALS

I think we've reached it. :)
Deprecate all rake tasks except `appraisal:all`

* Display warning when user invoke those tasks
* Run `appraisal` script when user invoke those tasks instead.