This change prepares GCS NIO to be merged into master, by making some
last minute API changes.

- Documentation has been added to many methods to clarify the preferred
  method for instantiating the library, which is the non-SPI version.

- Unit tests have been updated to not rely on the SPI, because there's
  no way to guarantee clean isolation of SPI usage across tests. We'll
  be relying on integration testing to test the SPI interface.

- The unit testing methodology has changed somewhat. FakeStorageRpc
  should be a private final field on the test class so, if desired,
  we'll be able to have the tests dip directly into fake memory.

- IOException has been added back to the throws of file system close, in
  case we decide to implement the "close all owned channels" thing into
  that method in the future.

- The getters on the configuration class have been made package-private,
  since there's no foreseeable reason they would be needed by the user.

- Injectable constructors have been added for Dagger 2 users.

In a future change, a README.md file will be added to replace the
documentation in the package-info.java file.