Add code + example for reading telemetry from hdfs

If you run a job on Telemetry data outside of the supported date range, you'll
get a helpful error message.
Update docs for Telemetry HDFS access
Move the 'mappertype' to job configuration.

This means that scripts do not have to implement an extra method to set the
mapper type, and it can just be set once in that data source's "setupjob"
function. This eases the burden for developing new scripts.

Any scripts already written to specify a mappertype function should continue
to work fine, as long as they reference one of the standard jydoop 'setupjob'
functions.
Merge pull request #41 from mreid-moz/telemetry_hdfs_support_clean

Telemetry hdfs support
Merge pull request #42 from mreid-moz/move_mapper_type_to_setup

Move the 'mappertype' to job configuration.