invoice: when displaying timestamps, use a configurable timezone intead of UTC

Regarding issue #15, this will allow us to change a configurable value to change invoice timestamps.

add to `panel2/panel2.conf`:
```
TIMEZONE = 'US/Central'
```
This also adds `pytz` to our list of requirements; installation can be done with pip:

```
sudo pip install pytz
```
or similar.

However, I'm still considering just making this relative-fuzzy dates like most other dates used, since obviously these timestamps will not always match the user's timezone. @puffrfish, however, wants 'actual' timestamps instead of relative ones to help staff members with debugging.