Handle null or undefined children

[gpoole]

Handles cases where <Animate /> is declared like so:

<Animate>
  {undefined}
  <div key='example'></div>
</Animate>

I've verified this with a test and in my own case, but please let me know if there's any additional testing needed to make sure this doesn't cause problems?

Fixes #7.

[coveralls]

Coverage decreased (-0.4%) to 74.182% when pulling 2f3e941 on gpoole:handle-null-undefined-children into e707fda on react-component:master.

[yiminghe]

can you make a complete test?

check whether classnames are added correctly

[yiminghe]

ok, thanks

[coveralls]

Coverage increased (+0.4%) to 74.912% when pulling 0a256c4 on gpoole:handle-null-undefined-children into e707fda on react-component:master.