INT-1197: Add support for WebSockets: client side

JIRA: https://jira.spring.io/browse/INT-1197

**DO NOT MERGE YET**

For me: this PR needs JavaDocs only.
All others (namespace, server, docs etc.) might be covered within other JIRAs.

@rstoyanchev, review this, please, too.
STS complains about this...

````
Description	Resource	Path	Location	Type
Type mismatch: cannot convert from Message<capture#2-of ?> to Message<Object>	PassThruSubProtocolHandler.java	/spring-integration-websocket/src/main/java/org/springframework/integration/websocket/support	line 70	Java Problem
````

Gradle's ok (java8 and java7)

`Message<?>` works ok.