Depend on ChefFS from Chef rather than knife-essentials.

This changes all requires to point at the version of ChefFS inside Chef
itself.  We also depends on Chef 11+.
Remove PATH reset code as it leads to errors in Chef 11.

It seems that setting the various _path variables to nil prevents
them from being generated automatically from chef_repo_path.

We depend on their automatic generation.
Restore path-clearing code using newer Chef::Config methods.
Chef ~> 11.8 required for fixes in chef_fs.

Specifically, older versions of Chef 11 fail when trying to create
already existing directories inside the backup.
Move path clearning inside begin/ensure block
Require chef/chef_fs/command_line to work around bug in chef_fs

Deep in chef_fs there is a missing require of the command_line library.
Require it here so that the restore doesn't fail.
Add --only-org option to knife ec restore
Log invitation upload error but keep going.
Add --skip-users to skip global user uploading.
Actually set Chef::Config values in auto-configure code.

This code should be factored out into a common method.  It was correct
in the backup script but incorrect in the restore script.

Fixing this allows knife ec restore to work without a config file in
the default case.
Add id to key export and call key import/export from restore/backup

Restoring the user id ensures that USAGs still work as expected,
provided that the user id is restored before the groups.

This commit adds --with-user-sql to call the key export/import
functions from the backup and restore commands.  Sequel is required
very late so the tool can still be used without it if desired.
Add --[no-]skip-user-ids option to support earlier use pattern.
Merge branch 'ssd/chef11-support'

Conflicts:
	knife-ec-backup.gemspec
Bump version to 2.0.0.beta.1 for development
Update README to reflect Chef 11 dependency.