Changed the HTTP connectivity from urllib to httlib

This enables persistent http connections (Keep-Alive) and is hence
slightly faster. Especially if you are behind a chain of web proxies and
update a lot of cells.
Removed the close() for HttpSessions.

The method was broken, not used & not tested. The sessions should be
closed when the process terminates anyways. If you need a close, because
you are running this inside of some container, it is easy enough to
iterate through the connections and close them all.
Update README.md

Add selecting worksheet by name example line
Merge pull request #74 from pcjonathan/patch-1

Update README.md
Update README.md
Set default returned value to empty string instead of None, #73
Merge branch 'master' of github.com:burnash/gspread
Added reference of how to get a list of all worksheets, #71
Corrected Two Factor Authorization note. Kudos to @tcorbettclark for the finding #45
Delete Makefile
This does not belong here
Fixed conflicts with @einvalentin's pull request
Returned to Python 3 support
Bring back Makefile
added targed for [Sphinx pages generation](http://blog.nikhilism.com/2012/08/automatic-github-pages-generation-from.html)
Fixed Makefile
Synced docs: removed duplicates with github's README
Added Analytics script
Fixed typo in Makefile
Changed link title