api keys in environment variables and helper class to do stuff with SimpleGeo Context results

[joncooper]

Hi there,

There are two things in this pull request:

1. API keys in environment variables SIMPLEGEO_TOKEN and SIMPLEGEO_SECRET. This is helpful for testing, and also for deploying to environments like Heroku. I prefer to keep keys out of the codebase.
2. A Context class, so that people can hang methods that ask questions about context results. My initial need was to do geographic resolution of IPs at a country level.

This is my first ever pull request so please excuse me if I've missed some point of etiquette here :)

Cheers,
Jon