Pig storage imrovements

Main improvements:

Build:
   * Update cdh4 build spec to target CDH 4.1.2

Pig:
   * Add new options to MongoStorage enabling handling of null values
et mongo special operations
   * Allow nested Bags and Tuples to be stored in Mongo
   * Allow Maps of scalars to be stored in Mongo
   * Transform Bags of Tuples with single scalar element to mongo
arrays
   * Parse ISODate date format in String scalars and store them as
mongo ISODate