Introducing `appraisal` CLI

This CLI is now a new way to use Appraisal to run your test suite

Before this change, Appraisal can only be invoked by `rake appraisal`, which has a bad side effect when you're trying to set it as a default rake task. Also, you cannot run a single test file without setting the environment variable yourself.

After this, you can pretty much run any command within a correct dependency context. For example, to run the whole test suite:

```
appraisal rake
```

To run a single unit test against `rails-3-1` appraisal:

```
appraisal rails-3-1 ruby -Itest test/foo_test.rb
```

Also, all the rake task has been changed into more sensible subcommands. From this help page:

```
$ appraisal help
Appraisal: Find out what your Ruby gems are worth.

Usage:
  appraisal [APPRAISAL_NAME] EXTERNAL_COMMAND

  If APPRAISAL_NAME is given, only run that EXTERNAL_COMMAND against the given
  appraisal, otherwise it runs the EXTERNAL_COMMAND against all appraisals.

Available Appraisal(s):
  rails-3-0
  rails-3-1

Commands:
  appraisal clean                  # Remove all generated gemfiles and lockfiles from gemfiles folder
  appraisal generate               # Generate a gemfile for each appraisal
  appraisal help [COMMAND]         # Describe available commands or one specific command
  appraisal install                # Resolve and install dependencies for each appraisal
  appraisal update [LIST_OF_GEMS]  # Remove all generated gemfiles and lockfiles, resolve, and install dependencies again
```