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
Merge pull request #1937 from bashtage/web-doc

ENH/DOC: Quick access to online documentation