Mutation.empty must be non-destructive [closes #22]

Mutation.empty must not erase the previous contents of DBObject, it should be a synonym for `noop`. However this change breaks backward compatibility and consequently requires major version increment
Merge branch 'master' into subset-2.0
1.1.x starts from here
refer version 1.1.0 in docs
dropping Scala 2.8 support
Merge pull request #24 from osinka/subset-2.0

Subset 1.1