Fixes #123

The golang.org/x/crypto/ssh/terminal package is deprecated and merely a
wrapper around golang.org/x/term. Thus, use the latter directly.

Set the terminal to raw mode *before* printing the prompt.
Otherwise the user (or the automated test) might enter the
passphrase before the terminal gets put into raw mode.

This is needed for some of the CLI tests to pass reliably in Travis CI.

This isn't actually a valid error since crypto.NewKeyFromReader()
handles re-allocating the buffer to a larger size if it fills up.

The commit changes how we get the username representation, and uses the
new pam API for checking the proposed login token.

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 command adds in the "fscrypt metadata" command. This command allows
advanced users to manipulate the metadata directly instead of just
creating a policy or protector as an option when encrypting a directory.

As some of these methods will require certain flags, error handling for
this case is also added. As the change passphrase method must indicate
when a old vs new password is necessary, additional KeyFuncs are added
which add this indicator.

Change-Id: Ibc92872088fae078df3c0eebd4f0cfcb7252d781

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