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.
Sets `APPRAISAL_INITIALIZED` when invoking command

This should be useful for people to track if their environment variables
has already been initialized and/or modified by Appraisal.
Update README

* Update examples to use `appraisal` command.
* Update Rakefile setting.
* Minor adjustment on version control policy.