Add just the most minimal alarm API

This is taken from Mehdi's PoC patch as a starting point.

blueprint alarm-api
Change-Id: If53a8332bdf6bd6bc727d37f5e6706db7e1f5ce8
Rename EventFilter to SampleFilter.

With the new Event tables entering ceilometer, having this EventFilter
class is going to cause confusion. Renaming this to SampleFilter.

Change-Id: I537ddaa79f1d28b4608518a2048be7c33c1b5acf
Doc improvements

Improve dbreco.rst based on suggestions from eglynn
Add remark about availability of compute meters in measurements.rst

Change-Id: I42cc3fe27cd9dab97c6c2353326c69577eb0f7cd
Merge "Add just the most minimal alarm API"
Merge "Rename EventFilter to SampleFilter."
Modify limitation on request version

Change-Id: I3b36945ceae4564de3804efbfd7ea0e4b637c85d
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "Doc improvements"
Merge "Modify limitation on request version"