this includes:
- Add Accumulator to the Start() function of service inputs
- For message consumer plugins, use the Accumulator to constantly add
  metrics and make Gather a dummy function
- rework unit tests to match this new behavior.
- make "flush_buffer_when_full" a config option that defaults to true

closes #666

This will basically make the root directory a place for storing the
major telegraf interfaces, which will make telegraf's godoc looks quite
a bit nicer. And make it easier for contributors to lookup the few data
types that they actually care about.

closes #564

Fixes #280
Fixes #281
Fixes #289

implement gauges, sets, counters

Currently adding with time is broken, because InfluxDB does not support
using precision for timestamp truncation both with and without
timestamps. This will be re-enabled once we fix InfluxDB to use the
precision argument for truncation in all cases, and a "unit" argument
in the line-protocol for adding points with non-nanosecond stamps

Fixes #175

This is mostly to solidify the overall structure with the agent,
plugins, and testing philosphy.