ACTIVEMQ6-6 - remove SPI usage

https://issues.apache.org/jira/browse/ACTIVEMQ6-6

Removed the jboss security implementations and also implemented the location of Transaction Managers using Service Loader and removed the jboss implementation.
Merging #37 on changing docs to MD
Merging #36 on SPI and TX integration
ACTIVEMQ6-9 - create html user manual for site

https://issues.apache.org/jira/browse/ACTIVEMQ6-9

The release profile now builds a static html version of the user manual for the web site using gitbook.

Also added a new profile 'distro' that builds the distribution but without the docs.

Updated read me with build process.