average_spectra returns standard deviation/standard error

- `plotter_helper.average_spectra()` now returns standard errors and standard deviations.

-  `Spectrum.errorfill()` which shades the error region.

- Adde example IPython notebook demonstrating new functionality
![image](https://cloud.githubusercontent.com/assets/19926/2818284/1308570a-cee3-11e3-8d36-6337be11d03f.png)