INT-3055: HTTP Outbound uri-variables-expression

JIRA: https://jira.springsource.org/browse/INT-3055

* Add to HTTP Outbound Endpoint `uri-variables-expression`
* Add mutually exclusive check for `uri-variables-expression` with `<uri-variable>`
* Tests and Docs

INT-3055: Fixes

* `ExpressionEvalMap` now can apply `Map<String, Object>`,
but the value must have a `String` or `Expression` type.

Polishing

INT-3055 Doc Polishing