Fix #56 (always-on recursion)

Always-on recursive application of all rebar commands causes too many issues. Recursive application is required for:
1. dealing with dependencies: get-deps, update-deps, and compile of deps right after get-deps or update-deps
2. projects with a riak-like apps/ project structure and dev process

The vast majority of projects are not structured like riak. Therefore, moving forward it's best to (by default) restrict recursive behavior to dealing with deps. This branch does that and also adds command line and rebar.config options for controlling or configuring recursion. Also, we introduce two meta commands: `prepare-deps` (equivalent to `rebar -r get-deps compile`) and `refresh-deps` (equivalent to `rebar -r update-deps compile`). riak-like projects can extend the list of recursive commands (to include 'eunit' and 'compile') by adding `{recursive_cmds, [eunit, compile]}` to rebar.config.

Fixes #56.