Implement "IPT" colourspace.

Closes #114.
Fixed docstrings.

Aligned first line of reference to next tab.
Instead of repeating the main reference in each function, it is now cited using standard reST citations
Fixed typo in function signature IPT_TO_XYZ -> IPT_to_XYZ
Added module methods to sub-package __init__.py file
Implements unit tests for colour.models.ipt module
Merge pull request #115 from colour-science/feature/ipt

PR: Implement support for "IPT" colourspace.