Rename DefaultSubject to PublishSubject

- https://github.com/Netflix/RxJava/issues/242
Rename from Operator* to Operation* to match convention
Merge pull request #244 from benjchristensen/pre-release-changes

Pre 0.8.0 Release Changes
0.8.0-SNAPSHOT
Gradle Release Plugin - pre tag commit:  'rxjava-0.8.0'.
Gradle Release Plugin - new version commit:  'rxjava-0.8.1-SNAPSHOT'.
Version 0.8.0