Pull an existing id from header, only generate a new id when needed.

Fixes #2 as well. Used Guava Optional and Supplier. I went with the
supplier after reading about urandom performance. Cleaned up the tests a
little as well.
Merge pull request #3 from service-enabled/pull-existing-id-from-header

Pull an existing id from header, only generate a new id when needed.
Specify compiler settings and use maven enforcer plugin

Fixes #4.
Add travis yml
Merge pull request #5 from service-enabled/pom-compiler-settings

Specify compiler settings and use maven enforcer plugin
Merge branch 'master' into add-client-filter