fix xpath for find_by_username

Update README.md

Fixing appropriately the "Sending videos" wait issue

@theCJMan pointed out that sending vids (in multiple contact loop) is to fast and then vids is not sent. We're adding a wait in each video "send_attachment" - in fact, it could be an issue with any large attachment. Tested this with 14MB video and multiple contacts, and it works nicely. 

For all contributors: please, make sure you're developing on the most recent version of the main repo!

updated the readme

updated messenger logout documentation

Merge pull request #29 from danm1-cmyk/newx

updated the xpath for input box for sending messages (2)

Merge pull request #18 from vadolasi/allow_custom_driver

allow custom driver

added extra info