Add support for java.util.Optional, reflectively.  If the class exists, we bind
it the same way we bind com.google.common.base.Optional.  Update tests
accordingly.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=77215848
Merge pull request #859 from google/moe_writing_branch_from_74b915b1eaa6de6b5efcf905b346b24ff563bbbb

Merge internal changes