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`.
I'll change that, but isnt that slower than just incrementing a counter? It has to recalculate the length everytime (unless lua keeps the length as metadata on the object).