Observable utilities

The main feature in this release is the addition of a suite of observable utilities, including mapPropsStream and componentFromStream. These are ported over from rx-recompose, which is now deprecated. Unlike rx-recompose, these utilities are compatible with any observable or stream-like library.

#196

Other changes

• createElement is now called createEagerElement. Also added createEagerFactory.
• flattenProp no longer omits the original prop from the resulting props.