Feature: Add ability to disable auto-finish

Typically when the bar reaches its total, you want the bar to complete
automatically, however there are cases where this is undesirable.  Now, the user
can pass through an `:autofinish` option which will allow them to disable this.

You can use it like so:

```ruby
progressbar = ProgressBar.create(:starting_at => 9,
                                 :total       => 10,
                                 :autofinish  => false)
progressbar.increment

progressbar.finished? # => false
````
Refactor: Convert '@finished' into an accessor

--------------------------------------------------------------------------------
Branch:        minad-master
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Refactor: Use 'fetch' instead of checking inclusion

--------------------------------------------------------------------------------
Branch:        minad-master
Change-Id:     I98f03470c971810aa7706dbbb6beefd65aa8ae76
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Style: Make whitespace changes
Docs: Add documentation about the new 'autofinish' feature

--------------------------------------------------------------------------------
Branch:        minad-master
Change-Id:     I1f7755746b0755a64ffe3b16f924d4f752f876f3
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Merge branch 'minad-master'