Update epslevel properly in PLUS() to avoid explosion.

Compare the following before and after merging this with development:

```
f = chebfun(@(x) sin(10*x));
Epslevel = zeros(100,1);
for k = 1:120
    f = f-.2*f;
    Epslevel(k) = f.epslevel;
end
semilogy(Epslevel,'.')
```

Before:
![before](https://cloud.githubusercontent.com/assets/2035010/3337536/f5567686-f84c-11e3-89da-03b6a3864d12.png)

After:
![after](https://cloud.githubusercontent.com/assets/2035010/3337545/23856e86-f84d-11e3-9a3e-11deb4748e9d.png)

P.S. I'm certain I fixed this issue before..

Edit: Image upload fail.