Add basic time module

[michardy]

This pull request would add a basic time module. The module provides two functions (utc-timestamp and local-timestamp) that return the current time in UTC or your local timezone as the number of seconds since January 1st 1970. The module uses chrono::offset to get the time in UTC and your local timezone and will add a dependency on chrono version 0.4.

If anyone would like to write a more complete time library I can help. This module is currently an minimal implementation containing only what I need.