Add copy method to PreparedRequest objects
Test cases fail when `HTTPBIN_URL` does not have trailing slash

Test cases can be run against a local httpbin server defined by
the `HTTPBIN_URL` environment variable, but it causes tests to
fail if the given URL does not end with a slash.

Ensure that the URL always ends with a slash.
Fix test cases that fail when running behind a proxy

When sending a request via `Session.send()` the proxies must be
explicitly given with the `proxies` argument.  This is not done
in the test cases, which means that they fail when run on a system
that is behind a proxy.

Update test cases to make sure the proxies are set in the sessions.
Merge pull request #1483 from dpursehouse/httpbin-url-without-trailing-slash

Test cases fail when `HTTPBIN_URL` does not have trailing slash
Merge pull request #1482 from dpursehouse/fix-testcases-with-proxy

Fix test cases that fail when running behind a proxy
Use urlparse.urljointo construct httpbin url
Derp. Use compat
Merge pull request #1486 from sigmavirus24/use_urljoin

Use urlparse.urljointo construct httpbin url
Better cookie docs on sessions.
Merge pull request #1490 from Lukasa/cookiedocs

Better cookie docs on sessions.
Comment markups, courtesy of @sigmavirus24
Document the Response.close() method.
Merge pull request #1476 from sigmavirus24/add_copy_to_prepared_requests

[2.0] Add copy method to PreparedRequest objects
Merge pull request #1494 from Lukasa/close

Document the Response.close() method.