PEP8 compliance and a faster adev() method

Hi, turns out allantools was exactly what I needed, but I didn't discover it until after I'd written my own code! After doing some benchmarks, the numpy-heavy method I wrote runs faster by about 60x than the allantools.adev() method, but allantools is far more complete. The new method is in leda_avar.py with a quick & dirty test / benchmark. 

I've also made some changes so that the code is closer to the PEP8 style guide (http://legacy.python.org/dev/peps/pep-0008/). The main motivation isn't pedantism, it's that docstrings can be viewed in iPython and also that they can be automatically converted into HTML/PDF documentation using Sphinx.