Fix JS Stomp.Client#connect raising an error when given multiple parameters

The callback (first parameter) were being passed on as the onConnect callback, which in the case of multiple arguments would be the username passed on signin. You can’t call a string.
Use the _errorCallback given in Stomp.Client#connect on final connection failure
Fix typo in HTTP transport error handler
Refactor _buildConnector by removing a bunch of code
Merge pull request #16 from Burgestrand/patch-1

Allow passing username and password to .connect() in JS client