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