Add cleanup call in Fetcher.java

Fetcher's inner class, FetcherRun overrides Hadoop Mapper's run().
Even though Nutch's FetcherRun doesn't need an explicit call the Mapper's cleanup() (Which is a blank function), it would increase the readability and completeness of the run Method to do so.
Ideally, every implementation of Mapper is supposed to do the following tasks:
1. Perform Setup

2.  Call map on the data set

3. Perform cleanups.

Moreover, in case a custom Fetcher is written extending Fetcher.java cleanup could get easily missed out.

PR for Fix: here