Added a standard way to change the registry used in unpickling operations.

Close #148
Change internal function run_pyroma to _run_pyroma
Added docstring and return value in test functions.
Fixed typos in comments and docs
Merge branch 'master' of git://github.com/infraredgirl/pint into develop
Removed incorrect text from the tutorial

Close #149
Fixed download links in documentation

Download links for ePub and zipped HTML were switched around.
Merge branch 'fix_links' of git://github.com/infraredgirl/pint into develop
Test clean up &, deactivated testing of multiplication/division for offset units

- Remove delta_kelvin in tests,
- Marked tests for multiplication/division of offset untis as known fail
- fixed test_issue66b which asumed that kelvin is an offset unit (which it is not anymore)
Make function _only_multiplicative_units more universal.
Reverted msg in Dimensionality error to old state.
Change: Unit definition creates OffstUnitConverter only if offset != 0 (and not for offset = 0.)
Improved to-method to work better with mixed units

"mixed units" here means a units with offset and units without offset. Now all
sensible conversions are possible. At most one offset unit per dimension is
allowed. Conversion from an offset unit to the delta_unit is also possible.

The latter breaks the addition/subtraction of quantities. A first workaround
for _addsub has been implemented which still needs further improvements.
Clean-up (by mistake missed in last commit)
Offset-Quantities: allowed subtraction, improved tests of inplace-operations

- Tests for new subtraction rule adjusted (before an error was raised)
- Implemented new subtraction rule (needs more work)
- Inplace-tests changed to use np.array (else the inplace operation is not
  performed!)
- Inplace-operations are not yet implemented; tests were deactivated.
- Fixed wrong names of units in unittests (use of plural in UnitContainer
  leads to invalid units)
Updated the contributing section of the docs
Updated CHANGES
Preparing release 0.5.1
Back to development: 0.6
Fixed tests for operation iadd and isub for quantities with offset-units

Need to be careful to use fresh (not yet inplace-modified) object in each
assert-statement.