Add ProofGeneral module

ProofGeneral is a frontend for Isabelle.  Adding this to personal branch
because it is unlikely to be useful for a larger audience anyway.
Disable three-window mode in ProofGeneral
Set path to main isabelle wrapper

Fix for current trunk of ProofGeneral
Set Isabelle command before loading PG
Replace file name expansion with concatenation
Only search for OS X if really on OS X
Fix string faces for Isabelle
Use expand-file-name instead of concat
Setup programming configuration in proof modes
Fix feature dependency

Proof mode is defined in proof-script.
Do not highlight overlong lines in proof modes

Due to unicode overlays the actual lines can be much longer than 80
characters, but the visual lines are quite short.  Hence disable the
irritating highlighting of overlong lines.
Update ProofGeneral