Add functions to add environment variables

This is my (newbie) attempt at some functions that add an environment variable to `/etc/environment`.

It lacks tests because I don't know how to do that.  However, I have tested manually (with `python -i mcv/env.py`) and it works fine (and idempotently).

One caveat: I'm assuming that the python process running this code will have write access to `/etc/environment`.  I don't know if that's a valid assumption, but there it is.
Whenever you see a pattern of constructing a list in a loop, a list comprehension is a good tool (and conventional):

http://carlgroner.me/Python/2011/11/09/An-Introduction-to-List-Comprehensions-in-Python.html

And now in Python you can also construct dictionaries in a comprehension

```python
>>> {k:v for k, v in [['a',10], ['b', 20]]}
{'a': 10, 'b': 20}
#