Add BooleanWritableConverter to use them in SequenceFileLoader in pig

[xuwenhao]

I have used BooleanWritable as a key or value in SequenceFile, since BooleanWritable is built inside hadoop, I suggest to have BooleanWritableConverter built into the elephant-bird.

[xuwenhao]

Would you please merge this to the code base? Or anything I else I could help?

[rangadi]

LGTM. @sagemintblue can we merge?

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