Add native only RoboPreferenceFragment. No equivalent in support lib. Solves https://github.com/roboguice/roboguice/issues/31
Fix for following during mvn release:prepare:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-release-plugin:2.0:prepare (default-cli) on project roboguice-aggregator: The artifact (org.roboguice:roboguice) requires a different version (3.0-alpha-2) than what is found (${project.version}) for the expression (roboguice.version) in the project (org.roboguice:astroboy). -> [Help 1]
[maven-release-plugin] prepare release roboguice-aggregator-3.0-alpha-2
[maven-release-plugin] rollback the release of roboguice-aggregator-3.0-alpha-2
[maven-release-plugin] prepare release roboguice-aggregator-3.0-alpha-2
[maven-release-plugin] prepare for next development iteration
Merge pull request #187 from stephanenicolas/preference-fragment

Add native only RoboPreferenceFragment. No equivalent in support lib. So...