Rename method count() -> countGroups()

Fixes gh-923
Add a PrefixMetricWriter interface to cleanly separate write/read

The PrefixMetricGroupExporter only really makes sesne if the writer
is aware of the groups, so it seemed better to use a new interface
than mix read/write.
Group resolution for RedisMultiMetricRepository with prefix

The prefix needs to be added before looking for keys. In addition
I rationalized the constructor and final fields (it didn't make
any sense for the prefix to be mutable).

Fixes gh-927
Rework HealthEndpoint and HealthIndicator

With this commit the state of a component or subsystem becomes a first-class citizen in Boot's application health support. HealthIndicators now return a Health instance with status and some contextual details.

An aggregation strategy has been introduced to aggregate several Health instances into one final application Health instance. Out of the box OrderedHealthAggregator can be configured to allow different ordering or a custom HealthAggregator bean can be registered.
Rename Status.status to Status.code
Return different http return status codes for different system health states

fixes #880
Merge branch 'rework-health-support'
Fix compile problem

Something slipped the recent merge
Add increment() to PrefixMetricWriter

In the redis repository we also switch to store the value in the
zset (so it can be atomically incremented) rather than in the
regular key-value.

Fixes gh-929
Remove List.sort usage for Collections.sort
Upgrade to Spring Security 3.2.4.RELEASE
Merge branch '1.0.x'
Improve redis repository implementations

Storing values in zset makes them less prone to races.

Fixes gh-929
Small docs tweaks in metrics
Align behaviour of in-memory and redis mult repos

The in-memory version has to force the incoming metric keys to
start with the group name. Redis doesn't have that restriction
but normally we expect both to be used in such a way that
the metric keys already match the prefix. In that case the two
repositories behave the same now in terms of set and get.
Update CONTRIBUTING.adoc
Fix bug in group names (again)
Add sources jar to default built
Polish
Extract AbstractHealthAggregator

This commit makes it easier for users to implement HealthAggregators