update readme with the latest changes
minor readme adjustments
rewrite the upload field to use the media manager from WordPress 3.5+

* Replace the old uploader code with code to use the new media manager. Uses code from Mike Jolley's tutorial: http://mikejolley.com/2012/12/using-the-new-wordpress-3-5-media-uploader-in-plugins/
* Enqueue media scripts so that the upload field works even when there is no TinyMCE/featured image on the current object
* Adds two new parameters to fields `upload_modal_title`, which sets the title of the button on the field itself and the modal title and `upload_modal_button_text` which sets the text for the button inside the modal
* Note: with this commit, the plugin now requires WordPress 3.5+

fixes #15, fixes #16, fixes #26, fixes #37
JavaScript improvements + replace chosen.js w/ select2.js

* update JavaScript to be up to standard with coding standards and be fully compatible with jQuery 1.9+
* replace chosen.js with select2.js and make select2 work for all select or multi_select field types
* Note: both the `chosen` and `select2` args will continue to work for assigning the feature to a field

fixes #45 and fixes #46
remove readonly string on upload button, that was a mistake
minor code standard adjustment in the js
remove blank line in the js
Add a clear/delete button for upload field

adds a clear button for upload fields, the text of which is configurable with the `upload_clear_button_text` field argument

fixes #56
unbreak datepicker fields
reformat css + add a few missing rules to prettify the UI

fixes #56
adjust readme to include changes from #56
add margin to textarea
add ability for groups to display their description

fixes #10
add group `required_cap` arg and use it to limit caps for groups

fixes #11
remove extra empty line
adjust readme with the last 2 enhancements that were added
minor adjustments in code load order

* move all DEFINEs and instance var filters into `admin_init()`
* get rid of `init()`
* get rid of `class_exists` check, it's not very useful

ref #25
convert plugin class to singleton

* convert plugin class to singleton which makes the code a bit cleaner
* keep the use of the global for backwards compatibility but we should remove it in a future version

fixes #53 and ref #25
fix a readme typo
fix another readme typo