Add a method to list available mappers

[martijn]

It's useful to see what is there in a BridgeRest? instance, so e.g. CyThesaurus? can present a drop-down box with options, instead of having to rely on insider knowledge.

A simple form could look like this:

GET webservice.bridgedb.org/contents

Name<tab>description
Human<tab>homo sapiens
Mouse<tab>mus musculus

The first column of course refers to the URL parameter to be used in subsequent requests. The second column could be simply a human readable description.