initial commit of dotplot

(This is a repeat of a PR I made a few hours ago, moving to a new feature branch per Josef's request)

This is a basic dotplot function that I think might be a useful addition to statsmodels/graphics.

The term "dotplot" has been used in various ways, these are dotplots in the style of Bill Cleveland's book, similar to what is implemented in this R package:

http://stat.ethz.ch/R-manual/R-patched/library/graphics/html/dotchart.html

Additional examples can be found in the appendix of this manuscript:

http://polisci.msu.edu/jacoby/research/dotplots/tpm/Jacoby,%20Dotplots,%20TPM%20Draft.pdf

The test_dotplot file (in statsmodels/graphics/tests) shows how it works with simulated data. These are the plots generated by the test file:

http://dept.stat.lsa.umich.edu/~kshedden/test_dotplot.pdf

Some examples with real data are here:

http://dept.stat.lsa.umich.edu/~kshedden/dotplot_gdp_example.pdf
http://dept.stat.lsa.umich.edu/~kshedden/dotplot_gas_prices.pdf

Scripts:

http://dept.stat.lsa.umich.edu/~kshedden/dotplot_gdp_example.py
http://dept.stat.lsa.umich.edu/~kshedden/dotplot_gas_prices.py

Comments and suggestions are welcome.