Erlang folsom poller

Add a folsom poller for erlang systems.

Sample output:
```shell
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.mem.total=27659640
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.mem.procs.total=10913264
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.mem.procs.used=10913264
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.mem.system=16746376
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.mem.atom.total=388601
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.mem.atom.used=371988
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.mem.binary=794584
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.mem.code=9970826
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.mem.ets=790728
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.stats.context-switches=74
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.stats.gc.num=11
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.stats.gc.reclaimed=27779
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.stats.io.input=1393
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.stats.io.output=2040
shh: 2014/11/10 10:03:50 when``=2014-11-10T10:03:50-08:00 sample#folsom.stats.reductions=10840
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.stats.run-queue=0
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.stats.runtime=0
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.stats.wall-clock=4999
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.drain.delivered=0
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.drain.dropped=0
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.message.processed=0
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#folsom.message.received=0
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#multi-poller.duration.folsom.seconds=0.002770
shh: 2014/11/10 10:03:50 when=2014-11-10T10:03:50-08:00 sample#multi-poller.duration.all.seconds=0.002796
```
You probably just want to use `Timeout: config.NetworkTimeout` instead of the custom ResponseHeaderTimeout / Dial func, etc. The semantics are a bit different, but it's now more standard.