irradiance.erbs behavior near zenith = 90

[wholmgren]

irradiance.erbs has a few issues when zenith approaches 90:

• kt blows up. This can be addressed using irradiance.clearness_index and a min_cos_zenith kwarg.
• dni blows up. This can be addressed with a max_zenith kwarg.

Similar to disc issues addressed in #540 and #400.

Versions:

• pvlib.__version__: 0.6.1