Allows user to change the wait timeout

Hi,
I've been using your gem to validate some of our legacy code. 
In my case, some of the functions took way more than 10sec.

The aim of the modification is basicaly to enable the user to change the timeout in a standard way, using RSpec configuration custom parameters.

I changed the handler.rb to check for custom parameter "wait_timeout" and update specs and documentation accordingly.

(I had to create a separate spec file, because I couldn't find the way to do it otherwise)

Cheers,
Sebastien
oops, read your comment in email and came to the PR, looking now