MapReduce needs to use Content-Type: application/json

When submitting a mapred job, the content-type needs to be set to application/json.
This fix is already present in the Basho version.
Update MapReduce.php to include a 2i search method

Added the indexSearch method from Basho's more recent version, while removing the Exception handlers from same.
Merge pull request #1 from jbsmith/master

use header "Content-Type: application/json" for sending map reduce jobs