Prefix subprojects with 'smack-'

instead of using the old baseName=smack appendix=project.name approach,
we are now going convention over configuration and renaming the
subprojects directories to the proper name.

Having a prefix is actually very helpful, because the resulting
libraries will be named like the subproject. And a core-4.0.0-rc1.jar is
not as explicit about what it actually *is* as a
smack-core-4.0.0-rc1.jar.

SMACK-265

experimental/build.gradle

@@ -1,11 +0,0 @@
-description = """\
-Smack experimental extensions.
-Classes and methods for XEPs that are in status 'experimental' or that should
-otherwise carefully considered for deployment. The API may change even
-between patch versions."""
-
-dependencies {
-	compile project(':core')
-	compile project(':extensions')
-	testCompile project(':core').sourceSets.test.runtimeClasspath
-}