Add onDuplicate callback

Having an onDuplicate callback allows the developer to customize what happens on the gui
and gives them the oppurtunity to perform any other specialized code.
Stop forms from being submitted on enter keypress

Forms with tags in them would be submitted if the user tried to
press enter to add a tag. Resolved this by using keypress and returning
false.
Update README with onDuplicate documentation
Merge pull request #6 from jfposton/fixes-and-improvements

Add onDuplicate callback and fix form submission bug