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](https://cloud.githubusercontent.com/assets/2035010/3337536/f5567686-f84c-11e3-89da-03b6a3864d12.png)

![after](https://cloud.githubusercontent.com/assets/2035010/3337537/fb708ff2-f84c-11e3-8fd1-1c59e25437b2.png)

P.S. I'm certain I fixed this issue before..