Deploying documentation to proper folder (#373)

What we're missing ATM is different documentation versions for different application versions. What this change does is that it's:

- finding out what is the current branch (e.g. 1.0.x)
- finding out out what is the name of the main adoc file (e.g. spring-cloud-sleuth)
- pulling the changes from gh-pages after checkout
- finding out what is the list of comma separated whitelisted branches (via the `docs.whitelisted.branches` prop) 
- in gh-pages creating a folder with name of the branch  (e.g. /1.0.x)
copying all the docs/target/generated-docs/ to that folder
- if the branch from which we're calling the script is NOT master then we're changing the ${main.adoc}.html to index.html so that it's easier to access the docs (e.g. http://cloud.spring.io/spring-cloud-sleuth/1.0.x/)

docs/pom.xml

@@ -13,6 +13,8 @@
 	<description>Spring Sleuth Docs</description>
 	<properties>
 		<docs.main>spring-cloud-sleuth</docs.main>
+		<!-- Comma separated list of whitelisted branches -->
+		<docs.whitelisted.branches>1.0.x</docs.whitelisted.branches>
 		<main.basedir>${basedir}/..</main.basedir>
 	</properties>
 	<profiles>