Improve withRouter

[agundermann]

This brings the current withRouter more in line with what we had in v3.

1. WrappedComponent: Lets people access the original component as a static property. Useful for unit testing the component without the router (might help with Rendering component wrapped in withRouter throws when no Router is present #4795). It's also what react-redux does.
2. wrappedComponentRef prop: Passed as ref to the original component. v3 and react-redux use a different API based on options: { withRef: true } and getWrappedInstance(), but this way seems more comfortable to me to use with ref functions, and we don't need to change the signature to something like options => Component => Component or (Component, options) => Component. It's also suggested in the react docs.
3. Hoist statics of the original component

[timdorr]

This is some good stuff for DX and usability of a withRouter component 👍

[agundermann]

Rebased to add some code to docs and edit CHANGES.md

[mjackson]

This looks great, @taurose :) thank you!

[adamroyle]

This change is now causing a warning about prop types in react 15.5 as it uses React.PropTypes instead of the prop-types package.

[timdorr]

Thanks, I'll see about getting that fixed up shortly. We did merge in some stuff after the PropTypes fixes, so there are some inconsistencies to fix.