ir2fr: easily calculate single-sided spectrum

* ir2fr to easily calculate single-sided spectrum
* neper_to_decibel and decibel_to_neper to convert between decibels and
nepers
Merge branch 'master' of github.com:python-acoustics/python-acoustics into ir2fr
Merge branch 'master' of github.com:python-acoustics/python-acoustics into ir2fr
Merge pull request #48 from FRidh/ir2fr

 ir2fr to easily calculate single-sided spectrum
Six: minimum version for readthedocs
Merge pull request #51 from FRidh/six

Six: minimum version for readthedocs
Update README
Merge pull request #52 from FRidh/readme

Update README
Improve documentation
Merge pull request #53 from FRidh/docs

Improve documentation
Imaging: Move list with ticks out of function.
Merge pull request #54 from FRidh/imaging

Imaging: Move list with ticks out of function.