Reveberation time from impulse response

All tests have passed limiting the `decimal` parameter of `assert_array_almost_equal` to 3. I'll need to investigate why with more decimals the tests presents slight differences in low bands.

This PR contains two wav files from [Openair](http://www.openairlib.net/) and a file with their (free) licenses.

Also, `t60_eyring` and others accept absorption coefficients in frequency bands so I changed the docstring.