The API uses enable_presto instead of is_presto_enabled now.

This change in the API was done to make it more consistent with other
options like enable_ganglia_monitoring etc. And seriously from the
user's perspective setting is_presto_enabled to true when they want
to enable presto looks a little weird.
Merge pull request #3 from mindprince/api-update

The API uses enable_presto instead of is_presto_enabled now.