Print some debug info if the solver does not support an expression.
Allow a MIP encoded Sum to take real valued offsets instead of just integer.

Includes regression test.
SAT encoding can add additional pigeon hole constraints for an encoding of AllDiff.
getTime for MiniSat and Walksat now returns the CPU time spent solving, excluding the time to create the CNF encoding.

By recording start time in the constructor, it was including the encoding time.
Add missing pigeon hole enum.
Slightly cleaner XKCD knapsack example.
Updated introduction doc.
Documentation for Task class.
Merge branch 'SATPigeonHoleAllDiff'
Simplify the output of help.sh for local_install so that the two lines can be copied easily.
Remove unnecessary -I from Mistral2 Makefile.