Numpy installation less verbose
Skip build and .tox directories in tests
Octave: Have certain methods available as seperate functions

* New functions
* Reference value is now module constant (soon we should merge these
module constants that are spread across the different modules)
* Rewrote properties using decorator syntax
Merge branch 'master' of github.com:python-acoustics/python-acoustics into octave
Merge pull request #34 from FRidh/octave

Octave