Add pyvenv-activate as a safe local variable and use it when loading dir-locals to activate a specific virtualenv.

I *think* this fixes #12.

It does use the pyvenv-activate variable first as it more explicit. For
example, I expected the code to do something like this:

  (defun pyvenv-workon (name)
    (pyvenv-activate (find-venv-path-in-virtualenvs name)))

With that in mind, specifying the pyvenv-activate just skips the look
up, hence it takes precedence.
Merge pull request #13 from ionrock/support-pyenv-activate-dir-local

Add pyenv-activate as a safe local variable. [
Add a bunch of tests, clean up the local file variable code.
Customization rework.

Different variables are now customizeable, some of them primarily
for add-file-local-variable or add-dir-local-variable.
Add keywords.
Add a mode menu with dynamic virtual environments.
Bump version to 1.3, update copyright.
use full basename (permit periods) in pyvenv-virtual-env-name
Merge pull request #15 from reece/master

use full basename (permit periods) in pyvenv-virtual-env-name
Set exec-path and path separator correctly on Windows
Merge pull request #18 from fabiocorneti/windows2

Set exec-path and path separator correctly on Windows
Do not check for when VIRTUALENVWRAPPER_LOG_DIR

Executing virtualenvwrapper hooks was protected by this check. I have
no idea why.
Use print function instead of statement.

This actually broke virtualenvwrapper functionality for Python 3.
Add pyvenv-tracking-mode to track the current virtualenv.

When this mode is active, pyvenv will adjust the current virtualenv
to the buffer-locally specified one after every command, adjusting
the current virtualenv every time the buffer changes.
Ignore ImportErrors when importing virtualenvwrapper.

When virtualenvwrapper is not installed, the hook loader would spam
the user with error messages. Don't do that.

Fixes #19.
Bump version to 1.4.