Rename `fact` to `fact_on` for uniformity

The DSL method `fact` was recently added, but most dsl methods take the
form of `<method>_on` for multi-node functions and `<method>` for
running on a default node. This makes `fact_on()` run on designated
nodes, and `fact()` run on the default node.
Merge pull request #78 from hunner/fact_on

Rename `fact` to `fact_on` for uniformity