checking for model feasibility following solve

[jGaboardi]

Following up from #237, we should probably be checking to the solution status following a .solve() attempt. This is because pulp does not raise an exception when model is not solved. However, a status code can be retrieved through the status attribute following a solution attempt:

LpStatus key	string value	numerical value
LpStatusOptimal	"Optimal"	1
LpStatusNotSolved	"Not Solved"	0
LpStatusInfeasible	"Infeasible"	-1
LpStatusUnbounded	"Unbounded"	-2
LpStatusUndefined	"Undefined"	-3

Basically, we'll want to check the status and raise an exception (or at the very least a warning) if the status is anything but 1. See here for a gist demonstrating an example.

cc @qszhao