Added support for testing with travis
Changes UnitRegistry to use instance variables.

This changes `_UNITS`, `_PREFIXES` and `_SUFFIXES`
to behave as expected.

Fixes hgrecco/pint#9.
Merge pull request #10 from bwanders/master

Multiple UnitRegistry instances without interference
Merge branch 'master' of git://github.com/hgrecco/pint

* 'master' of git://github.com/hgrecco/pint:
  Changes UnitRegistry to use instance variables.
  Added support for testing with travis

Conflicts:
	pint/pint.py