`buck query` for paths and deps backed by neo4j.

Summary:
Added a `buck query` command to query for all the dependencies of a target, or for all the
dependency paths from a target back to a source. The syntax is `T -?> S`
where `T` is the target, `?` is the search depth, and `S` is the source. Leaving `S` blank will
query for dependencies rather than paths. `?` can be a number or `*` for unlimited depth, and
an empty `?` defaults to depth `*`.

For example, buck query '//:android-binary ->' searches for all immediate dependencies of
//:android-binary.

Each time the command is run, a neo4j instance is spun up and the dependency graph information
is fed to neo4j. Then we construct a breadth-first traverser to crawl the graph depending
on the query. Adding more sophisticated queries should be straightforward given the neo4j
traversal API.

Test Plan:
Run the unit tests in `ant test`, which includes a new QueryCommandTest to test the new command.