Add sub example

Demonstrates how prototype can be used as a library and intentionally
follows a lot of what happens in the prototype test suite.  The idea
is to show that the same behavior applies. The name sub is a nod to
the 37signals/sub project which was at one point an inspiration for
comv, the parent of prototype.
Re-export PROTOTYPE_COMMAND variables as ${PROTOTYPE}_COMMAND

Helps preserve backwards compatibility with comv.  Could eventually be removed
-- I don't think this is the right way to approach this problem long-term.