Octave: several improvements

- Let certain methods call functions instead of duplicating their logic.
Include references to these functions in the docstrings.
- lower_frequency and upper_frequency now also have order=1 by default.
Integer -> Floats

At many places integers were used while floats should be used. Often
this works fine but not always, resulting in hard to find errors.

Changes:

- Integer -> Float in many places
- SOUNDSPEED module constant
- NC curves are now in a dictionary.
Merge branch 'master' of github.com:python-acoustics/python-acoustics into octave
Merge pull request #41 from FRidh/octave

Octave: several improvements
Turbulence: Improved plot layout
Travis
Using pytest in Travis
Re-organizing utils
Fix docs
Merge pull request #38 from felipeacsi/esum

Re-organizing utils
Merge branch 'master' of github.com:python-acoustics/python-acoustics into room

Conflicts:
	acoustics/core/descriptors.py
Merge pull request #44 from FRidh/room

Integer -> Floats
Moved core modules, rewrote third2oct

* Moved the modules inside core into the main namespace
* Rewrote esum. N-dimensionsal arrays are now supported. Acts like
np.sum()
* Rewrote third2oct. Now works for N-dimensional arrays. Performs the
summation along axis. If no axis is specified the highest dimensions is
used. Note that this might break backwards compatibility
* Renamed unittest folder to tests.

* Relative import for Python 3
Travis: Use cache for conda packages
Merge pull request #47 from FRidh/cache

Travis: Use cache for conda packages
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 pull request #46 from FRidh/utils

Moved core modules, rewrote third2oct
Merge branch 'master' of github.com:python-acoustics/python-acoustics into ir2fr
Travis: No longer test agains numpy 1.7
Merge pull request #49 from FRidh/travis

Travis: No longer test agains numpy 1.7