Initial support and test cases for error propagation
Extracted formatter from UnitsContainer
Square brackets are kept for dimensionality units (time, space, etc)
Add function to apply buckingahm Pi theorem to set of quantities
Added testing with tox
Exported formatter
Deduplicate units from candidates search

Previous to this commit, _parse_candidate was returning an iterable of
possible units, including prefixes and suffixes combinations.
If multiple values where returned, the calling functions were issuing
a warning to inform the user that there are multiple matches for a
given string. For example, if a user defines a new unit ms, it could
be mistaken with prefix=mili, unit=second. However, as the registry
stores also prefixed units many times a duplication is not a real
problem (eg. prefix=kilo, unit=gram and unit=kilogram).
To avoid issuing false warnings a deduplication step was added to
parse candidates. In the future, a more efficient alternative will be
to block adding colliding units to the registry.
Initial refactoring of the NumPy testing
Begin refactoring of the NumPy Testing infrastructure
Tox testing
In place conversion should not create a new object when source and destination units are equal
Merged master
Initial work to support non multiplicative units
Refactoring of the codebase

The pint module was split into different modules within the pint package.
The refactoring aims to provide better ways to test and extend pint.
In particular, it tries to separate code that does not require knowledge
of the registry. It also crete new classes to simplify the Quantity class.
Fixes and cleanups to the unit module

- Fixed future imports.
- Fixed parser to accept dimensions surrounded by square brackets.
- Added pi_theorem as a function from UnitRegistry
- Added value convert function to the UnitRegistry
- Added some documentation
Fixed NumPy tests to support comparison of Quantities and arrays
Implemented transformation of NumPy array content for ufunc call
Added future imports
Added "in" as an abbreviation for "inches"
Added "lightyear" as synonym for "light-year"
Added MPH
Added kph