Phs/mk 112 test helpers

Notable additions:

* Schemas are now "visitable"; it is easy for clients to iterate over them
* A new abstraction called `Tool` has been added.  A tool consumes a schema and can either describe a plan of action or simply perform it.
* The first tool is called `creator`, which calculates the difference between an existing HBase store and a schema, and applies the diff to realize the schema.  This handles tables, column families and coprocessors, with most (if not all) of the exposed configuration options.
* New spec helpers (exposed under the parent `hbase` helper) have been created.  These use the creator tool to build schemas from YAML (inline string or filename), and to query/mutate tables directly without lame hbase shell code.