Remove use of JSR 305 and dependency on com.google.code.findbugs

https://github.com/Netflix/RxJava/issues/192

The library and annotations were being used for trivial reasons so removing the usage so we don't have the dependency.
Merge pull request #211 from benjchristensen/issue-192

Remove use of JSR 305 and dependency on com.google.code.findbugs