This commit changes all the internal import paths from `fscrypt/foo` to
`github.com/google/fscrypt/foo` so that it can be built once we release
externaly. The documentation in README.md is updated accordingly.

Also, the README has a note noting that we do not make any guarantees
about project stability before 1.0 (when it ships with Ubuntu).

Change-Id: I6ba86e442c74057c8a06ba32a42e17f94833e280

This commit adds in the framework for adding commands and subcommands to
the fscrypt tool. This commit adds in the "setup", "encrypt", and
"unlock" commands. Additional information can be found by running:
    fscrypt <command> --help.

This commit defines how flags are parsed and errors are handled. It also
creates an extensible framework for prompting the user for information.

Change-Id: I159d7f44ee2b2bbc5e072f0802850e082d9a13ce