Model a frozen turbulent atmosphere

Calculate a snapshot of a frozen turbulent atmosphere.

Spectral densities have been implemented for Gaussian, Kolmogorov and
Von Karman spectra. Fluctuating fields can be calculated in 2D using
these three spectra. Wind and temperature fluctuations are supported.

1D and 3D fields are not yet implemented.

Only the classes that are used in the examples have been validated.
Add .png to gitignore
Merge pull request #9 from FRidh/turbulence

Model a frozen turbulent atmosphere
Patch to install matplotlib 1.3.1 with pip 1.5.x
Merge pull request #10 from felipeacsi/matplotlib-pip1.5

Patch to install matplotlib 1.3.1 with pip 1.5.x
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
Docs: docstrings updates to room acoustics module
Move constants into module namespace

These constants are commonly usable.
Docs: docstrings updates to reflection, turbulence, octave and power.
Docs: new structure
Merge pull request #16 from FRidh/docs

Docs: new structure and updated docstrings.
Merge pull request #17 from FRidh/bands

Move constants into module namespace
Turbulence: Split field generation from spectrum.