Fix template + option processing semantics

- When creating a new entry redefining a default definition, options
  replace default options with the same key.
- The special option {'--', Keys} removes given option keys from the
  default specification.
- Added 'sys' message handling to exometer_proc, which means that
  `sys:get_status(exometer:info(Probe, ref))` will work.
Merge pull request #93 from Feuerlabs/uw-template-options

Fix template + option processing semantics
- added {truncate, round} as option to histograms
- added system message handling in exometer_proc
- removed exo from deps (not used)
- bug fix in exometer_folsom_monitor
- exometer_report:reporter_pids() return only valid pids
- fix corner case when fetching highest values for histogram
remove unused local function
exometer_function: wrap single datapoint as list
Merge pull request #96 from Feuerlabs/uw-histogram-fixes

various fixes to histogram, exometer_report and folsom_monitor
Write gauges by default in statsd.

If a metric is not found to have a value in type map previously the
metric would be lost. This is a slightly unintuitive behaviour with
regard to the statsd reporter. Prefer instead to ship the untyped as
gauges, which has value in a statsd context.

Signed-off-by: Brian L. Troutwine <brian.troutwine@adroll.com>
fix datapoint type errors
Merge pull request #99 from blt/blt-gauge_by_default

Write gauges by default in statsd.
Merge branch 'master' of https://github.com/marksteele/exometer into marksteele-master

Conflicts:
	README.md
	doc/README.md
Merge branch 'marksteele-master'