Added query api to rest_api.md

Not sure what the url should be for list free nodes, open for
discussion.
Discussed this briefly on IRC with George.

It's ugly, but obvious what's going on. We're going to want to revise
all of this before too long anyway, I'm sure.
Merge remote-tracking branch 'moc/devel' into query-api
Implemented list_free_nodes

This required a slight fix to rest_call, allowing us to actually return
a meaningful response that's not an error.
Show node, and cli wrappers.
Wrote some tests.
Merge pull request #166 from zenhack/query-api

Started query api