Issue 8 basic validation

Added basic validation for form and JSON submission. Used the [voluptuous](https://github.com/alecthomas/voluptuous) library for this, which offers a pretty 
nice API for specifying the structure of Python data structures.

Additionally, tidied up views.py and the respective templates to avoid using a bizarre _additional_info function.

fixes #8 
short comment/example on this or the others here?