Split long lines

Logplex currently only handles lines < 10240 bytes in length.

This patch splits lines at 10000 bytes (so we have space for the syslog
header), recording each piece as another syslog frame.
Can't count on a single batch.Write() incrementing MsgCount by 1 anymore
a batch Write() != a single message anymore

So encapsulate a batches "fullness" inside of Batch.Full() and use a
boolean flag to setup the timer after the first write.
Set the timer before writting to the batch

The idea is that we don't know how much time batch.Write() will take, so
let's start our timer beforehand so that we account for that time as
well.
Merge pull request #19 from heroku/handle_looong_lines

Handle looong lines