Allows user to set and get environment mode the project runs in

Allows user to specify the environment when starting a server through the run --env param, eg:

$ meteor run --env=production

supports 3 environment modes:
- development: separate source files, cache busting, auto updates, etc.
- staging: behaves exactly like production but lets you demarcate code on client and server from production ready code, like your production analytics tracking code.
- production

Patch also reveals 3 properties on the Meteor object on both server and client, is_development, is_staging, and is_production to help facilitate environment demarcation.