Add ability to opt-in to int8 parsing

Switching the result of all COUNT operations to a string is
a pretty nasty breaking change, and the majority of us aren't
going to be hitting numbers larger than Number.MAX_VALUE

I bumped the major version number, but this makes the upgrade easier.  I know in my apps I don't ever use 64bit numbers but I do COUNT(*) all over the place.

fixes https://github.com/brianc/node-postgres/issues/378

to opt in just do

```js
require('pg').defaults.parseInt8 = true
```

Basically this is a handy shortcut  for this:

```js

require('pg').types.setTypeParser(20, 'text', parseInt)

```