+ can: Proxy Support, fixes #102

I did some updates according to the discussions in #102.

- added ```ConnectionType``` to ```HostConnectorSetup``` with the following options:
 - ```Direct```
 - ```AutoProxied``` (default)
 - ```Proxied(proxyHost: String, proxyPort: Int)```

- ```HttpManager``` now calls ```connectorFor()``` which then delegates to ```hostConnectorFor()``` or ```proxiedConnectorFor()``` in case a proxy should be used. ```proxiedConnectorFor()``` returns a ```ProxiedHostConnector``` which fixes URIs and host headers and delegates to ```HttpHostConnector``` which connects to the proxy and is shared between ```ProxiedHostConnector```s to the same proxy.
- added ```Uri.normalizedForHttp()``` to DRY up the default port logic.