Prefix function names with spring-

* Add `spring-` prefix to function names

Fixes: #9

This commit renames each sub-module in the common, consumer, function
and supplier groups with a prefix of `spring-`.

* Update README.adoc links to new prefixed names

function/spring-splitter-function/build.gradle

@@ -0,0 +1,3 @@
+dependencies {
+    api 'org.springframework.integration:spring-integration-file'
+}