Fix external links in Scaladocs.

This addresses issue #41, by just taking the path of least
resistance and just entering explicit URLs for the three
external references we currently have.
start a scout if remote leadership fails

when handling a leadership change, it is possible that sirius can fail
to resolve a remote reference to a new leader. This has been observed
during rolling restarts and on commodity hardware. If this happens and a
scout is not started, the node's leader will never resolve (ie remain
unknown) and will be unable to participate properly in its cluster.
Return false for timed-out `isOnline` calls

There's an ask involved in resolving isOnline, which, though it seems
unlikely, could possibly time out. If this happens, we don't want that
AskTimeout exception to escape and blow things up. So, call a timeout
"false", and let the polling take care of the rest.
Updated external documentation links.

* Link specifically to Akka 2.2.1 apidocs for Agent.
* Point to Akka 2.2.1 configuration docs, which are more useful than
  the (nonexistent in 2.2.1) NettySSLSupport class's apidocs. :)
add periodic state check for leader state

Under certain rare conditions the Leader can be in a corrupted state
which takes some time to resolve. A periodic state check has been
added for the following:

* if the elected leader is unknown, a scout is started
* if the elected leader is remote, but there is no leader watcher, a
leader watcher is started
Add better timeout test
Merge pull request #83 from tbarker9comcast/tweak-leader

Start a scout if remote leader resolution fails
Merge pull request #84 from tbarker9comcast/periodically-check-leader-state

add periodic state check for leader state
Do not send out-of-order events to StateSupervisor

PaxosStateBridge was erroneously sending unhelpful events to the
StateSupervisor, due to a nextSeq bookkeeping bug. They were dropped
correctly, which is nice, but there were lots of errors, and some extra
work being done. This fixes that error.
Merge pull request #82 from comcast-jonm/scaladoc-warnings

Fix external links in Scaladocs.
Merge pull request #88 from clinedome-work/goodbye-badnewsbears

Do not send out-of-order events to StateSupervisor
Merge pull request #87 from clinedome-work/isonline-timeouts

Return false for timed-out `isOnline` calls
Augment MembershipActor monitoring

Add monitor hooks for current cluster.config membership, paths in the
last resolution attempt, time since last resolution attempt, and the
number of resolution failures the actor has encountered.
Merge pull request #91 from clinedome-work/membership-monitoring

Augment MembershipActor monitoring
prune dead members from memberAgents

dead members are pruned from the membershipAgent based on the following
formula:

```
allowedPingFailures * pingInterval + pingInterval / 2
```

This makes dead membership handling more proactive than just checking
the cluster config