+ can: proxy support, fixes #102
- added ConnectionType to HostConnectorSetup with the following options:
  - Direct
  - AutoProxied (default)
  - Proxied(proxyHost: String, proxyPort: Int)
- added Map[String, ProxySettings]to ClientConnectionSettings (proxy settings per scheme)
- 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 ProxiedHostConnectors to the same proxy.
- added Uri.normalizedForHttp() and Uri.httpScheme() to DRY up the default scheme/port logic
- changed the param order in Uri.toEffectiveHttpRequestUri()
+ http: add method to supply default headers to HttpMessage
+ can: add defaultHeaders to HostConnectorSetup