changed samplerate to sample_rate for compatibility with pysoundio
fixed bug in __len__ and __getitem__

I missed a few instances of the old seek_start and seek_end methods, which were superceded by seek_absolute
Slices with a start bigger than an end are fixed automatically
added note that libsndfile must be installed
changed file_mode to mode for consistency
fixed example in README.rst
V0.2: minor api tweaks and bug fixes

updated setup.py to reflect changes.