Create CatchupActor, CatchupSupervisor

Reworking catchup process. Mutate window size based on historical
success/failures, similar to TCP slow-start / congestion-avoidance.
These actors provide the basis for this, and included here are the
config options we'll need. Nothing's hooked up.

Also included are the new messages it'll use.
Wire in new catch-up algorithm

Wire the new catchup actor and supervisor into place. Includes reworking
a *lot* of the way the PaxosStateBridge works, but hopefully putting it
in a much more understandable state than before. Also includes changing
SiriusPersistenceActor so that it uses the new LogSubrange methods.
Remove old catch-up remnants

Remove GapFetcher*, and the messages we used for sending around
subranges of logs. Don't need 'em anymore!
add contributors.md file
Merge pull request #76 from tbarker9comcast/create-contributing-page

add contributors.md file
Add MBean monitors for window, ssthresh

This stuff will be very helpful to have accessible through JMX. You
could conceivably detect network badness by a lower-than-normal
ssthresh, and it might be fun to graph window. Just suggestions, YMMV.
Simplify Catchup Request logic, more config

Eliminate CatchupActor in favor of ask(). In addition, make more of the
catchup params configurable, and add some explanatory documentation to
the CatchupSupervisor.
Merge pull request #75 from clinedome-work/rework-catchup-algorithm

Rework catchup algorithm