Create lamassu-transactions-csv

Exports contents of postgres transactions table to .csv, encrypts with user's PGP key, sends to email address.

**Dependencies:**
- postfix
- mutt
- software-properties-common (?)
- python-software-properties (?)

**Lacks:**
- Install script for above dependencies and importation of PGP key (probably should be part of lamassu-install, with PGP import optional)
- Calculation of commission earned in fiat at time of transaction (should be part of lamassu-server/lib/postgresql_interface.js, correct?)
- Relative path for `transactions-*.csv` file