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 
Not a bad idea—need to look into it, because there may be 1-2 things that are a little gnarly with PEP 8. In the meantime, back down to 3 lines (PEP 8 says 2, but some other PEP or guide says 3…I tend towards 3).