Expose Mutation.writeToWAL in .put(), .delete(), and batch()

Various data manipulation methods on Table and Batch instances now
support a 'wal' argument to influence whether the mutation is written to
the Write-Ahead Log. This is only supported in recent HBase version.

Fixes issue #36.
Update NEWS
Expand docs for the 'wal' arg for Batch.put() and .delete()