Implement Connection Management

[swissmanu]

various issues (like #30) asked about how a connection management (lost connections etc.) could be realised.
though orchestra-jsapi shows a way how this can be implemented (https://github.com/swissmanu/orchestra-jsapi/blob/master/index.js#L41), it makes use of internal APIs of the underlying XMPP client.

since the question appeared repeatedly, it might make sense to implement a minimal connection management mechanism directly into harmonyhubjs-client.

[hufftheweevil]

I've found a very simple fix to this issue. Please see PR #42