Enable support for multiple fields

Fixes #8.

The 'clickable' argument in Dropzone JS requests a HTML Element or a (jQuery) selector. This selector is not relative to the Dropzone. The option was set to 'button', resulting in each Dropzone instance registering itself to every button in the document. 

The fix looks for a 'button' inside the Dropzone element and provides the actual HTML Element as 'clickable' argument.