We should always log the descriptor not the entire policy structure.

Chaning the --user flag to (optionally) check for a proper keyring setup
allows us to fail early in cases where we need a working keyring.

The --user flag can now be used to have the targe user (the one whose
keyring and password will be used in fscrypt) be different than the
calling user. Very usefull for things like

	sudo fscrypt purge /media/joerichey/usb --user=joerichey

which will now have privileges to drop caches, but will properly clear
the keys from the user's keyring.

Almost all actions only need to to check that the fscrypt metadata
exists (this is handled by the Mount methods). Only "fscrypt encrypt"
need to be sure the filesystem also supports encryption, so this check
is added.

Protectors are only reverted if they were created, and Policies are only
depovisioned on failure.

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 "fscrypt metadata add-protector-to-policy" and
the "fscrypt metadata remove-protector-from-policy" subcommands. These
commands allow for the creating of policies protected by multiple
protectors.

Change-Id: Id7e6c057448d15757c838a82d487a1b9806f585d