Add PostgreSQL support ;)

[zoujiaqing]

Very practical project!
When will PostgreSQL be supported?
I'm really looking forward to it.

[schveiguy]

Thanks! Almost all the work that has gone into this is based on my need for it in a mysql project, which is why it's in the best state.

Having someone who needs it for postgres would help. My next push was going to be for sqlite, because it's easy to test.

I also need to finish the docs, and tinker with the API. I still don't love how some of these things work.

[schveiguy]

@zoujiaqing what is the postgresql D library you use?

[zoujiaqing]

I am using hunt-entity for it.
But linking so slow.
I hope use native libpq.so to accelerate conpile.