ENH: Allow exponentially weighted functions to specify alpha directly
- Closes #10789
- Passes Travis CI, nosetests, and flake8
- Covered by new tests in test_window.py:TestMoments
- Documented in computation.rst and v0.18.0.txt

I am not sure we do any validity checks on any com parameters

eg is alpha=0 valid?
 That's actually a question I had. Zero alpha is not okay, but zero half-life isn't either, and the code is not checking for that. Should I add the checks for `0 < alpha <= 1` and `span >= 1`, `com >= 0`, `half-life > 0` following from that?
 yep I think we need some sanity checks as well here - raising a helpful ValueError would be nice (rather than giving weird/odd results)
 thanks @evectant just minor comments.
ping when pushed and green.
 @jreback: Added the comments, slightly reworded the description in computation.rst. Pushed and green.
 looks good. just some minor documentation comments.
 pls rebase and push again, master has been updated
 @jreback: Pushed and green.
 thanks @evectant great PR! 
 give a look at the built documentation (should be done shortly), http://pandas-docs.github.io/pandas-docs-travis/ and make sure looks ok (obviously where you changed things!). If not pls issue a follow.
 @jreback The docs look good to me. Thanks for your help with the commit.
 looks good

thanks for the PR!
