Improved stability and bug fixes

This release adds a limit to the number of MPMD task groups that can be bundled (issue #11). Launching jobs consisting of too many groups can cause ALPS, schedulers, and other support and resource management tools to hang. For the sake of system stability, the number of separate MPMD tasks is limited to 2048. However, an arbitrary number of splits is still possible.

This release also fixes a bug where the executable argument passed by keyword to the API would fail to appropriately convert raw strings to a valid internal representation.