Add file_polling_input and decoders/filters for system stats

This PR includes a new file_polling_input input which periodically takes
the contents of a file and puts them into the payload of a heka message.

It also includes decoders and matching filters for parsing system memory stats
from `/proc/meminfo`, cpu stats (load avg, and processes) from `/proc/loadavg`,
and disk stats from `/sys/block/$DISK/stat`.
fyi: l.P not necessary https://hekad.readthedocs.org/en/latest/sandbox/index.html#lua-parsing-expression-grammars-lpeg