Refactored the pow() tests so that they can be used with math.pow() in test_umath.py later.
umath.pow() now tested like the built-in pow().
Added test.
Fixed test.
All test_uncertainties.py tests pass. This includes the fixed test on x**p where p = 3.1±0.1.
Fixed derivative. The case 0**(y < 0) never used the derivative because wrap() first calls the function on floats, which raises an error. However, it is easy to return a correct derivative even for this case (like done with this patch).
umath.pow() now has the same derivatives as AffineScalarFunc.__pow__. I am not fully sure that this is correct, as the two functions differ slightly (http://stackoverflow.com/a/10282852/42973).
Added test for corner case of built-in pow() and math.pow().
More tests pass.
Removed duplicate tests.
All test pass. This includes the new tests for umath.pow: tests on its derivatives in corner cases, and tests of the exception raised in corner cases.
More correct error message.
Adapted test to Python 2.5.
Removed duplicate test (see test_uncertainties.py:test_power_special_cases)
Fixed incorrect version test (non-working version).
Removed unnecessary test (the exception is raised, so no result is returned and the standard deviation is not used). Tailored the test for Python < 2.6.
Tests pass with Python 2.5. All other versions remain to be tested.
Merge branch 'math_pow_tests' into math_pow_tests_py2.3

Conflicts:
	uncertainties/__init__.py
Adapting the syntax to Python < 2.5.
Fixed typo.