* Set VIRTUAL_ENV enviroment in activate_this.py

VIRTUAL_ENV was not set in activate_this.py, while set in other activate scripts.

* Add tests for activate_this.py

* Fix syntax in test_activate_this.py

* fix

* more robust testing for the activate this

* fix Jython support of activate this

* added instructions for changing --system-site-packages in existing environment

In case someone forgets to use the --system-site-packages flag and wants to change it later, it's currently hard to find out how, so I added instructions here.

* added explanation of cdsitepackages command

the command is from virtualenvwrapper

* removed virtualenv reference

* removed trailing spaces

* initial version of xonsh activate

* partially add activate.xsh to virtualenv.py

* Add base64-encoded xonsh script

* actiavte.xsh

* some initial tests

* fixed deactivate argument error

* added source command to xonsh test and added newline to the end of activate.xsh

* remove powershell refs

* add xonsh to testing requirements

* black tests/activation/test_xonsh_activation.py

* need raw string to escape paths in windows

* some path safety fixes

* put in some debug for tests

* black the test

* change import order

* added docs

* @gaborbernat feedback

* better python version constratints for xonsh

* black

* xonsh install version includes minor

* refactor need_xonsh

* removed unused import

* removed blank line

Universal newlines are deprecated as of python3.5, which makes the
following warning be emitted when deprecation warnings are enabled:

  .tox/py35/lib/python3.5/site.py:165: DeprecationWarning: 'U' mode is deprecated

Saying "...there's no need for activation" is incorrect if a Python script spawns subprocesses or otherwise relies on `PATH` and `VIRTUAL_ENV` environment variables being modified in the same manner as the activation scripts.

An example of this becoming an issue is Ansible inventory scripts end up using the non-virtualenv Python binary when Ansible is installed within a virtualenv--which is the approach Homebrew uses to install Ansible.

Replaced pypi.python.org with pypi.org in the download URL.
Added `-L` option to `curl` to follow redirects from randomized hash
URLS.

Also, replaced pypi.python.org/pypi with pypi.org/project throughout the
docs.

This fix addresses the most important part of #1042.

Be explicit about where `activate` is and maintain the convention used throughout.

Update userguide.rst