Feature wronskian

A couple of examples:
```
>> op = @(x,u) diff(u, 2) + (u);
L = chebop(op, [-pi, pi]);
x = chebfun('x', [-pi, pi]);
w = wronskian(L, cos(x), sin(x));
norm(w - 1, inf)
ans =
     2.065014825802791e-14
```
And
```
>> op = @(u) diff(u,2) + 4*diff(u) - 5*u;
dom = [0, 5];
L = chebop(op, dom);
x = chebfun(@(x) x, dom);
g = exp(x);
f = exp(-5*x);
w = wronskian(L, f, g);
% Compare with the exact wronskian:
norm(w - 6*exp(-4*x), inf)
ans =
     3.754152373280907e-13
```