Support relevancy condition for fields on case detail screen

Ability for a field element in suite.xml to have a "relevant" attribute that, if it evaluates to false, will not show that field. Implemented for growth charts (FogBugz 127286), to allow app to show different graphs for female vs. male patients.

Sample usage:
<field relevant="current()/gender = 'male'">

Goes along with case-list-fields-relevancy branch in commcare-odk.