Use token prefixes for creating report

From discussion with @mlavin: By using prefixes, users won't have to remember an order to send things in, e.g.::

    nutrition report patientid wt=10 ht=50 muac=10 oedema=y

There should be some balance between what's easy for us to parse and what is easy for users to do. The current system (no prefixes) is easy for us but difficult for users. Above is in between, but even better would be users don't have to use an equals sign in between each token name and token value.

We might consider using <a href="http://pyparsing.wikispaces.com/">pyparsing</a> or <a href="http://www.dabeaz.com/ply/">ply</a>.