Changing syntax to specify selenium browser version

Simplifying syntax to run tests with selenium webdriver.

Old syntax:

`venus run --test spec.js --selenium --selenium-server 192.0.0.101 --selenium-browser chrome`

New syntax:

`venus run --test spec.js --selenium 192.0.0.101 --browser chrome`

Additionally, you can now specify browser version:

`venus run --test spec.js --selenium 192.0.0.101 --browser chrome|20.0`

The old syntax still works, for backwards compatibility.