Added power-tripping tool

Added `powertrip.py`: the tool you'll love to hate. It performs basic sanity-checking of the repo, flagging:

- missing ``__init__.py`` files in library/test code
- code that has execute permissions set
- Cython code that doesn't have generated C code paired with it (checks that a C file of the same basename exists and isn't empty)

Here's some example output (these aren't real failures in the repo; I messed with some of the files):

```bash
$ ./powertrip.py 
Directories missing init files:
    skbio
    skbio/core/tests
    skbio/math/tests

Library code with execute permissions:
    skbio/core/sequence.py
    skbio/core/distance.py
    skbio/core/alignment/ssw/ssw.h
    skbio/math/stats/tests/__init__.py

Cython code missing generated C code:
    skbio/core/alignment/ssw/ssw_wrapper.pyx
    skbio/math/_subsample.pyx
```

There's a lot more features we could add, but wanted to get this very basic version under review. It's also hooked up to Travis.

Fixes #446.
I know we don't want major performance gains in this script but having `skip_dirs` as a `set` would be better for the type of lookup happening in the `_validate` method. Also, should these two directories just be appended to whatever other directories are passed?