Change hashbang in bin/p to remove reference to rvm.
Show source when inspecting or using to_string on a function.
Added exception handling.  Still could use some thought, probably a refactor.
In this implementation raise parses and operates much like return.  try /
rescue / ensure are used for handling exceptions.  rescue binds the error
object (anything wrapped in P.error) to the environment and executes it's
block.  It's up to the programmer to check what the error is and handle it or
re-raise it.
Bug fix:  there was an indexing error when saving lines of history for the
repl.
Several repl bug fixes and minor improvments (around loading / saving history
and reporting exceptions).
Added --interactive option.