Added rolling counters interceptor, which counts the frequency of
objects extracted from Flume events in a sliding window. The object
counts are appended to the events header.
An example of how to perform a rolling topN using Flume. Includes two
Interceptor implementations: RollingCounterInterceptor and
TopNInterceptor.

RollingCounterInterceptor: extracts and counts items from Flume Events
in a sliding window style. With configurable length and granularity.

TopNInterceptor: takes the counters emitted by
RollingCounterInterceptor and computes the topN

A custom source, PeriodicEmissionSource, connects to the TopNInterceptor
and periodically emits the topN list.

The example given in flume-topn-example.conf is "trending topics": the
most popular hashtags seen in twitter status updates over the past X
minutes. The latest topN list is emitted every X seconds.
Added .gitignore