Integrate TestRPC with Truffle

[gnidan]

• truffle test should use an in-memory instance of the testrpc by default, unless a separate test environment is specified.
• truffle migrate should include a new flag called --dry-run or -d that allows you to use the TestRPC to fork from the latest block and run your migrations in a sandbox, as if they were running on the real chain
• The default TestRPC configuration used in both cases should be configurable from truffle.js (aka truffle-config).