upgrade to guava 17.0

It has the jsr305 (javax.annotations) dependency as transitive
now so we have to declare it explicitly.
This is mainly to coordinate the dependency version with downstream projects.
Merge pull request #613 from groldan/2.6.x_to_guava_17

upgrade master to guava 17.0
Merge branch 'master' of git://github.com/geoserver/geoserver