ENH: added parameter to use periodic x-coordinates

Using the example given in the current `np.interp()` and changing the intervals to be interpolated to:

    xvals = np.linspace(-np.pi, +np.pi, 50)

will lead to the following result:

![figure_3](https://cloud.githubusercontent.com/assets/3889819/4417109/b4c39028-454e-11e4-801f-cbe59c244e3b.png)

note how the proposed enhancement allows a proper interpolation. The only change is the addition of a `period` parameter:

    yinterp_new = interp(xvals, x, y, period=2*np.pi)

It is actually `None or float`, right? You can take the docs for `np.sum`'s `axis` parameter as an example of how that is typically done, see [here](http://docs.scipy.org/doc/numpy/reference/generated/numpy.sum.html). You should probably add some explanation on it defaulting to `None` which means non-periodic interpolation, and mention that the value should be different from zero.