Calendar DateTime BIFs

[KronicDeth]

We'll have to decide whether we implement these and tie them to web APIs or make calling the web APIs explicit.

• [x] date/0 (#336)
• [x] localtime/0 (#335)
• [ ] localtime_to_universaltime/1
• [ ] localtime_to_universaltime/2
• [ ] now/0
• [ ] posixtime_to_universaltime/1
• [ ] system_time/0
• [ ] system_time/1
• [x] time/0 (#336)
• [ ] time_offset/0
• [ ] time_offset/1
• [ ] timestamp/0
• [x] universaltime/0 (#334)
• [ ] universaltime_to_localtime/1
• [ ] universaltime_to_posixtime/1

[bitwalker]

I would prefer to have our platform abstraction layer (i.e. liblumen_core/sys delegate to Web APIs transparently (where possible) to implement built-ins, it would still be possible for one to call Web APIs directly, but in general we'll need to support the BIFs anyway in order to support legacy code/arbitrary packages off Hex.