Add distinct_permutations function
Add distinct_permutations to api.rst
Fix indents and add docstring for helper function
Put new function in alphabetical order

Addresses review comment:
https://github.com/erikrose/more-itertools/pull/36#discussion_r10687235
Properly format arguments in docstring

Addresses review comment:
https://github.com/erikrose/more-itertools/pull/36#discussion_r10687277
Don't use capital variable name

Addresses review comment:
https://github.com/erikrose/more-itertools/pull/36#discussion_r10687292
Replace collections.Counter for 2.6 compatibility

Addresses review comment:
https://github.com/erikrose/more-itertools/pull/36#discussion_r10687247
New class not needed for single testcase

Addresses review comment:
https://github.com/erikrose/more-itertools/pull/36#discussion_r10687329
Spurious argument in 1449f87

Addresses review comment:
https://github.com/erikrose/more-itertools/pull/36#discussion_r10687329
Make tests pass under Python 3.x.

Comparing ints and strs is not allowed.
Make a few style tweaks: no behavior changes.

This is a teensy bit faster, likely because it doesn't have to set the value twice.
Add distinct_permutations to version history.
Merge in `distinct_permutations()`. Close #36.
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