Implemented <select> field type

Fields can now take a fourth (optional) parameter during register()
calls.

This parameter, indexed as 'meta', is an array.  For selectboxes, it's
implemented to use key/value pairs. (For example: `$my_options['USA'] =
'United States'`)

---------------------------------------------------

Sample usage inside a register() call:

```php
$fields = array( array( 'name' => 'lang',
'type' => 'select',
'label' => 'Language',
'meta' => array( 'en' => 'English',
'es' => 'Spanish',
'fr' => 'French')
));

$args = array( 'label' => 'Files',
'post_type' => array( 'page' ),
'filetype' => null,
'note' => '',
'button_text' => __( 'Attach Files', 'attachments' ),
'modal_text' => __( 'Attach', 'attachments' ),
'fields' => $fields);

$attachments->register( 'my_custom_instance_name', $args );
```
Attachments_Field is now an abstract class

No need for an Implementation when an Abstract class will suffice. New custom fields now only need to extend Attachments_Field.
Updated documentation
Added search() method to allow searching for Attachments based on keywords or other attributes #16
Killed the min-height and animated the margin/padding that was making the 'Remove' animation jumpy. Fixes #26.
Initial implementation of select field along with data structure for implementing it. Current issue: multiple selects don't pass along all selected values.
Added 2 more configuration options: position (meta box position: normal, side, advanced), priority (meta box priority: high, default, low, core).
Added styles when Attachments meta box is placed in side.
Implemented select field. Updated documentation. #29
New parameter for Attachments attributes methods. You can pass the index (int) of the Attachment you'd like to utilize when firing the method.
Version bump for 3.2. Props @trovster @dannewns. Huge props and thanks to @cpommiss for the initial legwork on the select field!
Version bump for 3.3. Props @trovster @dannewns. Huge props and thanks to @cpommiss for the initial legwork on the select field
Merge branch 'master' of github.com:jchristopher/attachments
Merge branch 'side' of github.com:kasparsj/side
Modified the CSS a little bit to accommodate some hidden borders. Added a 'Toggle Fields' link to save on space.
Moved the toggle link within the thumbnail container to save even more room. Some further CSS refinements.
Updated documentation for new meta box positioning. Closes #30. Thank you @kasparsj!
Stable tag bump for wordpress.org
Forgot to remove the select field from the roadmap
Fixed typos
Fixed a markup flub