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)

Don't use backticks `` ` `` to enclose variable names in error messages. That is used in docstrings so that variable names are rendered as code in markdown. You probably want your error messages to match as much as possible the ones in `compiled_interp`, which do not enclose argument names in any form of quotes, see [here](https://github.com/numpy/numpy/blob/master/numpy/lib/src/_compiled_base.c#L553). I also think that the leading "Argument" is not needed, i.e. make the message `'period must be non-zero'`, but that's up to you to decide.