Make normal add/sub use normal, not in-place, ops

In some cases, normal and in-place operations can give different
results. Now, when an operation is done on a Quantity, the exactly
corresponding operation is done on its magnitude, instead of always
using the in-place operation.
Make iadd_sub and add_sub private to _Quantity

Rename iadd_sub to _iadd_sub, and rename add_sub to _add_sub. These
methods should not be publicly exposed. Instead, users of the _Quantity
class should use operators.
Fix compatibility with python 2.6
Add test for issue #93
Add test for pull request #94
Correct symbol translation for inches and feet

get_symbol('inch') should return 'in' not 'international_inch'.
This seems be a definition error.

This commit provides the updated definition and related test.
Updated CHANGES
Revert "Fix compatibility with python 2.6"

This reverts commit 3418786aa70c0c36a1f330c0df0bf59a2361fa7c.
Replace set comprehensions, dict comprehensions and set literals
Add NullHandler
Fix tests to run on python 2.6
Fix numbered positions in string formatting operations ({} to {0})
Better replacement for dict comprehensions in python 2.6
Fixed conflict
Fixed deps in tox.ini
Make sure that kwargs keys are str (for Python < 2.6.6)
Python 2.6 support in docs and CHANGES
Updated AUTHORS.
Merge branch 'test-issue-93' of git://github.com/jturner314/pint into jturner314-test-issue-93
Merge branch 'fix-add-sub' of git://github.com/jturner314/pint into jturner314-fix-add-sub