Fixed proper handling of verbosity and output shape for rio merge

This fixes two bugs in ```merge```:

* ```verbosity``` was not always defined, and thus broke tests once they were created
* the shape of the destination image was hardcoded to always have 3 bands regardless of input number of bands

This also adds tests for 100% coverage.  Includes a test fixture for generating test data files; we may want to consider this pattern for other tests as well.

I'm getting flakey results from travis-ci for a simple array equality test; some python versions pass sometimes and others do not.  It should be an all or nothing proposition.  All tests pass locally (Linux Mint 17, Python 2.7.6).