Add support for a custom PhantomJS logger 

Related to this issue: https://github.com/jonleighton/poltergeist/issues/94

Allows the user to specify a IO object for the output of PhantomJS. This allows the testing, capturing and silencing of the output. 

- supports any `IO` compatible object
- defaults to `$stdout`
- specified via `:phantomjs_logger` in `Driver.new`