More work on fixes for these plotting bugs.

1.  Fixed problem whereby the limits on the plot for f = chebfun(@(x) x,
    [-Inf, Inf]) were not symmetric, making the plot look strange.

2.  Renamed some variables in @unbndfun/plotData.m for clarity.
Two plotting tests are added:

1. A test on the symmetry of the x-lim and y-lim of function f(x)=x.
2. A test for #1274 - plot of a complex constant.