Disable fparser debugging functions by default

And fix the fparser Makefile so that whether they're enabled will
depend correctly on the configure options
Set the FUNCTIONPARSER_SUPPORT_DEBUGGING #define.

But only if configured with --enable-fparser-debugging.
Add summary line if fparser debugging is enabled.
Merge pull request #253 from libMesh/fparser_debugging_off

Disable fparser debugging functions by default
use autodiff fparser objects
Generate derivatives, compute gradient and dot, factor out setSpacetime()
use exceptions to deal with errors in AutoDiff
add has_derivatives() method, CHANGE RETURN STATUS of AutoDiff()!
rename setSpacetime -> set_spacetime