Preparing release 0.4.2
Back to development: 0.5
Make it easier to run tests on installed Pint.
Started to use uncertainties

See #6, #24, #76, #77
Removed unnecessary import
Improved travis file
Fixed test. uncertainties knows how to deal with a measurement being substracted from itself.
Removed unnecessary code from _Measurement
Removed unnecessary method form Measurement.
Adds another units library to faq.rst

- Adds udunitspy
Add excluded env combinations to .travis.yml
Extract out formatting rules into separate dictionaries.
Fix https://github.com/hgrecco/pint/issues/112 and add _DEFAULT_FORMAT
Split off the formatter into its own file.
Move the constant arguments for formatting to formatter.py
Move the constant arguments for formatting to formatter.py
Fully parse the spec, fixing https://github.com/hgrecco/pint/issues/111
Merge branch 'rec-issue-107' of https://github.com/rec/pint into rec-issue-107
Started to write unit tests.
Finish tests for new condensed formatting mode.