Changed plugin prototypes.

Adding underflow, remainder_numerator, and remainder_denominator
arguments so that no information is lost when doing subtraction or
division operations. Consequently, all arithmetic plugins must be
completely redone.

Also changing results-dir plugin into the 'set' plugin. The 'set'
plugin will be used to set the value of an variable. Therefore, it
can still be used to set the value of the 'results-dir'.

Removed 'version' plugin because all the version numbers will be
available as variables.