Merged. Thank you Jendrik!

Now that we have support for the "internal" scripts, do you think we can merge 
this, Gabi?

Apart from two really minor things, the code looks good so far but still lacks
support for the "internal" scripts. This should be easy to change.

I split the issue of DEBUG flags off into issue498.

The code for the other flags is done in a first version and Gabi agreed to review 
it.

These are intentionally per-module variables. You usually only want debug output
for one module at a time (the one you're debugging ;-)), so separate options
like "--debug-fact-groups", "--debug-simplify" and "--debug-translate" would
make more sense than a general "--debug".

An alternative solution would be to use a configurable logging mechanism with
subloggers for this. This wouldn't be difficult to set up with the
infrastructure that Python has and would immediately give us some nice
additional benefits. For example, it would immediately give us a "debug flag"
for each module that doesn't currently have one, without cluttering the option
space.

I have started working on this. One question is whether we want the DEBUGs 
module- or translator-wide. At the moment translate.py, simplify.py and 
fact_groups.py define DEBUG.

The translator has a bunch of options that can only be set by modifying the code
right now (flags in translate.py; MAX_CANDIDATES and MAX_TIME in
invariant_finder.py). These should be exposed to the command line.

While we do this, we should also document what the preferred way to disable
invariant synthesis is (right now, it is setting MAX_CANDIDATES to 0.)