[WIP] New Mink WebAssert class for SPAs.

This introduces a new WebAssert class that you can use in situations where you have a JavaScript based apps where you don't have the normal browser 'user action' -> 'http request' -> 'page load' loop.

It essentially performs the assertions in a loop and will retry the assertion for a period of time.

I wanted to gauge any potential interest so no tests at the moment, sorry!
these methods are not meant to be part of a public API (an assertion failure is throwing an exception, that's all)