Java DSL: Lambdas for Namespace Factories

* Add `Function` and `Consumer` copies from Java 8
* Rework some functional interfaces to `Function` or `Consumer`
* Introduce `Channels`, `MessageHandlers`, `MessageProducers`, `MessageSources`, `MessagingGateways`
- some convenient wrappers for Namespace Factories to be used from Lambdas
* Add Lambda-methods for those new wrappers
* Provide some refactoring
* Upgrade to Boot `1.1.7`

Java DSL: Lambda_Factories

* Change all `IntegrationFlows` methods to `.from()`
* Introduce internal `Function<?, ?>` extensions for particular cases, e.g. `MessageSourcesFunction`.
Now end-user has to cast Lambda parameter to concrete type to get desired factory, e.g.:
```
return IntegrationFlows.from((MessageProducers mp) -> mp.imap("imap://user:pw@host/INBOX"))
```
* rename `.fromFixedMessageChannel` to `.from()` with additional boolean flag
* Introduce `PollerFactory` Lambda for `.poller()` EIP-method on `EndpointSpec`
* Move `Pollers` stuff to the `.core` package to fix package tangle
* Refactoring for some code style
* Fix some typos

spring-integration-java-dsl/build.gradle

@@ -33,7 +33,7 @@ ext {
 	mailVersion = '1.4.7'
 	slf4jVersion = '1.7.7'
 	springIntegrationVersion = '4.0.4.RELEASE'
-	springBootVersion = '1.1.6.RELEASE'
+	springBootVersion = '1.1.7.RELEASE'
 
 	linkHomepage = 'https://github.com/spring-projects/spring-integration-extensions'
 	linkCi = 'https://build.spring.io/browse/INTEXT'