Update to use Elixir DateTime module with time zone suppport

[andrewtimberlake]

I have updated both the parser and render to use Elixir’s DateTime module
This allows us to correctly parse time zone offsets in timestamps
The renderer still accepts erl timestamp tuples, assumed to be UTC
If a DateTime is supplied, and it has time zone information, that will be used in rendering.
Parsing now returns DateTime objects in UTC based on the supplied time zone offset of the timestamp (a breaking change)

I have also fixed compile warnings.
This moves the minimum Elixir requirement to 1.10

[bcardarella]

This is great :)