Feature trigcf

This is an implementation of the polynomial CF method for periodic functions. Here is an example:

```
>> fh = @(x) a(1)*sin(exp(cos(x))) + .01*a(2)*cos(12*x) + a(3)*cos(2*x) + a(4)*cos(x) + a(5);
>> ff = chebfun(@(x) fh(x), [-pi, pi], 'periodic');
>> nt = 12;
>> pf = cf(ff, nt);
>> plot(pf-ff, 'r')
```
![untitled](https://cloud.githubusercontent.com/assets/890964/3656394/d4b68144-1185-11e4-9914-1b63e44dd4f2.jpg)