Work on the concolic mode.

This has added what turned out to be a very tricky part: matching up the FormFields on the page and the symbolic variables. Currently this is a huge hack, and we should discuss how this should really be dealt with. I suggest that there is some modification to SymbolicSource so that it can track the exact element which a given variable came from, or at least some easier way to match them up after-the-fact.

Also, it still does not run on the simple-validation-3 example, because the traces cannot be merged correctly. I will look into this now.
Add symbolic branch conditions to the terminal trace printer.
Fixed bug in merging when reaching trace ends.
Added option to control tree graph output in concolic mode.