Support om 0.7.1 (ctor -> descriptor)

This upgrades to om 0.7.1 (React 0.11.1), mainly fixing mixins due to 0.7.0's
breaking API change.

*   Generate descriptors instead of constructors for components with mixins.
    *   Generated symbol suffix changed from $ctor to $descriptor
*   Change :ctor key to :descriptor key
*   Fix tests to use React.isValidComponent (React 0.10.0 changed what component
    objects look like)

Bumps to the next minor version due to breaking API change from changing $ctor to
$descriptor.
Update CHANGELOG for 0.3.0
Merge pull request #32 from Prismatic/om-0.7.1

Support om 0.7.1