Use the CombineFileInputFormat to avoid too many mappers

Change the SequenceFilePailInputFormat to use the CombineFileInputFormat. This should reduce the number of input splits for Pail sources. In my tests, several thousand splits were reduced to one.

There is an issue with this change. It will not work with the hadoop 2.0.5-alpha, which is the version of hadoop that I have deployed. The reason is that the implementation of CombineFileInputFormat in that version does not call listStatus(JobConf conf) from the mapred package to get the list of files. Instead it calls ListStatus(JobContext conf) from the mapreduce package.

I fixed this by pulling in CombineFileInputFormat to avoid version conflicts.
You can see my comment in a previous change about this. I think it would be much better to upgrade the hadoop library, but not sure how to fix otherwise. Also not sure what that upgrade would mean for other users of this library. 

https://github.com/nathanmarz/dfs-datastores/pull/45