Modifications per recommended changes.

* Document Scope as not offering thread-safety
* Remove synchronized sections within Scope
* Restrict withName() and withSubScope() to only allow names matching
  [A-Za-z0-9.][A-Za-z0-9_.\\-]*
* Consolidate tensorflow and tensorflow_op into one BUILD target
* Fix mistake in opname in Javadoc example
* Change method qualifier in inner test classes to default
* rename baseName -> opPrefix, baseOpName -> opName