add discussion and examples of Angstrom and Kasten formulas to docs

[mikofski]

Related to issue #269 and a follow on to PR #278 which adds the Kasten pyrheliometric formula to calculate Linke turbidity factor given broadband AOD and precipitable water, the Bird-Hulstrom broadband AOD correlation and Angstrom formulas to calculate alpha exponent from AOD at two wavelengths and to calculate AOD given one measurement and alpha.

This issue proposes to add a brief discussion and some examples to the clear sky documentation. The discussion can also mention other methods not currently included in pvlib-python.