ENH: Quick access to online documentation

Adds a simple function to the api which allows the online documentation to be
accessed in the default browser.  Has three modes:

* Default: open the documentation page
* String: search for the input
* Function or class: go directly to the generated documentation
Changed name of doc to open_doc
Rewrote to be testable
Added method to choose stable or devel, with default to look at version
Added tests
Added compatibility code for Python2/3 urlencode
Renamed open_doc to webdoc
Fix bug preventing history from being returned
Fixed typo
Fix printing of random effects names in summary
Minor optimization for no_smw case
Optimize a bit by precomputing ex_re^2
DOC: Rename Change README type to rst

Improves landing appearance on github
Refactor parameter handling, improve starting values
Restored steepest ascent, with some modifications
Restored steepest ascent, with some improvements
Add todo about unequal group sizes
Use MixedLMParams to specify free/fixed structure
Override from_formula so that extra array args can be passed by name
Test maintenance
Merge pull request #1942 from bashtage/readme-format

DOC: Rename Change README type to rst
Merge pull request #1937 from bashtage/web-doc

ENH/DOC: Quick access to online documentation
Merge pull request #1940 from kshedden/lme-docstring

REF: refactor and speedup of mixed LME
DOC: GEE docstring rst error