Add new Client constructors

This patch changes how Clients are created. It adds the following
functions:

* `Client::from_local` - use the specified root version from the local
  repository as our initial trusted root.
* `Client::from_pinned_root_keyids` - use the specified root version,
  threshold, and keyids to trust a root fetched from the local or remote
  repository.
* `Client::from_pinned_root_keys` - use the specified root version,
  threshold, and public keys to trust a root fetched from the local or
  remote repository.
* `Client::from_pinned_root` - use the specified root metadata as the
  initial trusted root.

This deprecates the old constructors:

* `Client::new`
* `Client::with_root_pinned`

Closes: theupdateframework#229

Author: erickt