WIP preliminary docs, impl for the slave rtype

** confusion over the general idea of a slave, vs. a slave on a
   particular master.  Is masterid part of the message?  Part of the
   slave object?  Or do we add a way to enumerate the slaves on a master
   and the masters for a slave?
preliminary docs for the log rtype
preliminary docs for the logchunk rtype
Add a step resource type and DB table

TODO:
 * define 'identifier' type:
   * [a-zA-Z0-9._-], <50 chars
   * add a runtime verifier for it, too
   * still handled as unicode internally
 * model
 * migration script + tests
 * db connector + fake + tests
 * data API + tests
 * add 'steps' to the build type
WIP