Cache repeated calls to get_min and get_max in Expression.get_domain().
Tests for SAT encoding of inequality predicates.
Fixes for SAT encoding of various inequality predicates.
Conjunction.
Interface to Gurobi.

Squashed commit of the following:

commit 9df506c96f36c95b7d1b8fab604fbbe7efc00d5e
Author: 9thbit <9thbit@gmail.com>
Date:   Tue Jun 18 16:18:04 2013 +0100

    Move Gurobi interface to available_interfaces.

commit 0e8f0d235692758e96d0d61c08051bf092876e50
Author: 9thbit <9thbit@gmail.com>
Date:   Tue Jun 18 16:10:21 2013 +0100

    Allow to set Gurobi node limit.

commit 67132d72a5520790ef16fe548df43299a76803b2
Author: 9thbit <9thbit@gmail.com>
Date:   Mon Jun 10 17:56:51 2013 +0100

    Add Gurobi install instructions to README.md and some other small changes.

commit 7b0e0ac6543dd7357b332db09e813913eb37d671
Author: 9thbit <9thbit@gmail.com>
Date:   Mon Jun 10 15:01:36 2013 +0100

    Use the environment variable GUROBI_HOME which should be set during installation.

commit e415f0e7d3ece63c41e66e9f8d54ee0a386ec1b3
Author: 9thbit <9thbit@gmail.com>
Date:   Mon Jun 10 11:59:49 2013 +0100

    Work in progress native Gurobi interface.