ENH: added audio dimension checker (Fixes #147)

Well, I thought this would address a bunch of problems, but some of them seem to have been fixed in the meantime, or I was just wrong when I was trying to do some things in the past.

Anyway, I added functionality for fixing the dimensions of audio data. When I went to add it to various functions, the only one that needed it was ```convolve_hrtf```. If I'm wrong about that then it will be easy to add it to the beginning of any other functions that handle audio data later.  
OK. Will add the ```axis``` parameter, as it obviously does need to be there.