Change Travis build to do releases

Add a tag_release.sh script to the repo and get it to verify that the version number exists in the root readme tables.
Change travis build to produce a sha256 hashe of each generated pack zip
Make the travis build extract the pack name from the git tag e.g. core-xml-schemas-v2.0 => core-xml-schemas
Use the pack name to locate the generated artifacts (and their hashes) and put them in a new dir
Get the travis releases plugin to pick up these files and release them to github

gchq / stroom-content