Fix interaction with radio buttons

#12 The ```check``` function selects the element from the form based on the 'name' attribute.  This is fine in most cases, but not in the radio button case, where multiple radio buttons will have the same 'name' attribute but you only want to check one of them.  

I've updated the ```field-to-selector``` function to return an enlive selector based on the 'value' attribute if the element is a radio button.

I've also upgraded the library to use clojure 1.6.0