Log strophe messages to console when debug mode is on

Log errors at appropriate level. This mostly addresses https://github.com/candy-chat/candy/issues/248#issuecomment-50351725, with the exception that the original exception's stack trace is lost. This is [Strophe's fault](https://github.com/strophe/strophejs/blob/master/src/core.js#L2219-L2222) and we can't fix it ourselves.

See #248.