remove unneeded dependencies from nio

mziccard noticed the following with google-cloud-nio:
1.  org.mockito:mockito-core dependency should have test scope
2.  google-cloud-nio now depends on the whole google-cloud, we should depend just on google-cloud-storage
3.  To make 2. effective we need to also remove google-cloud dependency from google-cloud-contrib main pom.xml, we can let each child module pull its own dependencies.
