Merge branch 'master' into test_power_funcs
Merge branch 'master' into test_power_funcs

I keep the tests on power operators that were removed from the master branch (so that it passes tests and can be released).
Merge branch 'master' into additional_tests
Applied (manually) update for (-1)**9.1 from master branch.
Merge branch 'additional_tests'

Conflicts:
	uncertainties/test_umath.py
	uncertainties/test_uncertainties.py
Added tests for Python 3.
Merge branch 'master' into additional_tests
Merge branch 'release' into additional_tests
Merge branch 'master' into additional_tests
Merge branch 'release' into fix_power
Merge branch 'master' into fix_power
No exception raised anymore with 0**ufloat(0.3, 0.01).
To do: handle the case of "zero uncertainty" * "NaN derivative" => 0 std dev.
uncertainties.ufloat(0, 0)**0.5 now returns 0 instead of raising ZeroDivisionError. This is better.

0**ufloat(0.3, 0.01) returns 0±NaN instead of raising a ValueError: math
domain error. However, this is not perfect: a more reasonable result would
be 0.
All power tests pass, now.
More modern exception raising.
Removed Python >= 2.6-only test (just to see why Python < 2.6 shouldn't have the tests).
Put back Python 2.6 test.
Simpler code layout.
Merge branch 'release' into fix_power
Merge branch 'master' into fix_power