add postgres support

[zilkey]

I'd like to use experimental, but I'm on postgres. It does not seem to be trivial to convert the existing mysql functions to postgres, but I have a version that seems to be equivalent.

The ruby version is:

top_8 = Digest::MD5.hexdigest("#{experiment_name}#{user.id}")[0...16]
top_8.to_i(16) % num_buckets

The SQL version is:

select abs(('x'|| substr(md5('experiment-name' || id),1,16))::bit(64)::bigint) % 2 from users;

Are there any plans to include different database adapters in this gem?

[zilkey]

Please disregard the code above. The sql does not match the ruby.

To support ruby, could we change the algorithm to something simpler? Like, having a random seed number in the config file, then just adding that random number to the user's id? (an idea from @dlikhten)

That way, the math would be super simple in the database, and would be database agnostic.

[dlikhten]

Less math = always my favorite approach. :+1: