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.
Good idea! I'll update the default to be `None` (since sets are mutable). I'll also update `ExecPermissionValidator.extensions` to be a set.

> Also, should these two directories just be appended to whatever other directories are passed?

I'd rather let the user have complete control over this since there might be cases where a `data` directory *should* be validated (likely not in skbio, but maybe if this code gets generalized it could be used in other projects). Okay to keep these as defaults, but not append them to what the user supplies?