Introduce Medium class for PSTD
Change name of module/project to 'acoustics'
Use FFTW FFT if available in Atmosphere()

The FFT of FFTW performs much better than fftpack that is used by
default in numpy and scipy.
Several changes to Atmosphere
- Improve unit tests.
- Improve several method names
Merge pull request #13 from FRidh/fft

Use FFTW FFT if available in Atmosphere()
Merge pull request #14 from FRidh/docs

Change name of module/project to 'acoustics'
Merge branch 'master' of github.com:python-acoustics/python-acoustics into atm

Conflicts:
	acoustics/atmosphere.py
fix path to file for atmosphere test
Merge pull request #15 from FRidh/atm

Several changes to Atmosphere
Merge branch 'master' of github.com:python-acoustics/python-acoustics into pstd
PSTD