Let user specify the id of the input bar

Input bar id is not globally set, but passed to the directive
through the input-id attribute.
Move activeBufferChanged listener to WeechatCtrl

I don't believe that the inputBar directive should know about the
mobile cutoff we have set for GB. We should aim to make it as
reusable as possible
Merge pull request #185 from cormier/fix-input-bar

Fix input bar