PUT and DELETE for _config/admins.

Out of necessity, this commit also adds DELETE for any configuration
key.
set_config -> put_config; remove JSON encoding/decoding.

This is a pretty big change to the configuration client, but the old
method wasn't workable:

1. The double-JSON encoding hack didn't actually work for configuration
   values that were strings.  This made it impossible to set admin
   passwords via set_config.
2. Nothing else is called set_*, and the operation's a PUT anyway.
Update changelog.
Bump version.