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
Fix #7 -- always use real division.
Merge pull request #12 from dbrnz/real_division

Fix #7 -- always use real division.
Merge remote-tracking branch 'upstream/master'