Support writing datetime columns

[theroggy]

Closes #58

[jorisvandenbossche]

Nice!

[brendan-ward]

Thanks for working on this @theroggy !

Please also update docs/known_issues.md to note that it is now possible to write datetime values, but only at ms precision. Also probably a good idea to note that some drivers do not support full datetime and may instead store only date values (e.g., shapefile); we may further want to note that if needed to store the original values, they should encode them to a string (it seems the recommendation to store full datetime values is to put them in a string)

[theroggy]

Thanks for working on this @theroggy !

Please also update docs/known_issues.md to note that it is now possible to write datetime values, but only at ms precision. Also probably a good idea to note that some drivers do not support full datetime and may instead store only date values (e.g., shapefile); we may further want to note that if needed to store the original values, they should encode them to a string (it seems the recommendation to store full datetime values is to put them in a string)

Done.

[brendan-ward]

Thanks @theroggy , this is great to have in place for 0.4.0