Add interpolation functions.

I've implemented functions for interpolating sets of points. You can find detailed description on [wiki](https://github.com/nbeloglazov/incanter-interpolation-examples/wiki/Incanter-interpolation).

If you accept this pull request I'd like to move wiki page to incanter's wiki. 

I also implemented few small apps that use `incanter.interpolation` visualise 2d and 3d interpolations. You can find it [here](https://github.com/nbeloglazov/incanter-interpolation-examples).

Thank you,
Nikita