Add unparse_url to urllib3.util.url

This round-trips with parse_url (or at least equivalently by the RFC).
Add url attribute to Url, which gives the same thing as unparse_url()
Improve code coverage
Remove unparse_url, in favor of Url.url
Make something a little clearer
pyOpenSSL: Fix WantWrite issue #412
Added requirements of GAE
code formatting changes
Minor change
Sentence changes
code formatting changes
removed the http and add requirements also in README.rst
minor change
Removed the duplicate and add in Contributions list
Removed the extra line
updated the contribution file.
Merge pull request #460 from Twitterapi4all/gae-doc-security

Added GAE requirements doc in security.
Added 'Python with SSL enabled' section and merged AppEngine into it.
Formatting.
Formatting.