Changed computation of centering parameter

Introduced max and min for centering parameter

Introduced max and min for step size (all now defined in ecos.h)
fixed bug in computing mu, this increased reliability a lot
Fixed re-formulation in dimacs problems (Sedumi Dual --> ECOS primal)

DIMACS test script significantly changed - comparison to sedumi

safediv-ed the cone module (all divisions in cone.c are now safe)

added test to iterative refinement module to quit if error goes up too much

added backtracking/exit strategy if linear system cannot be solved accurately enough (this quits
then with the exitflag ECOS_NUMERICS)

adjusted mex file accordingly

+ many little changes here and there
added error computation for dimacs problems (dimacs_error.m)

small bugfix in runecos.c
fixed merge in run_dimacs_test