godoc

This pull request cleans up the [godoc documentation for pq](http://godoc.org/github.com/lib/pq). It adds documentation for all methods where useful, and adds an example to the package declaration.

It also changes the code to not export several internal types and variables. In particular:

 * `pq.Open` is only called from `drv.Open`, clients don't call it directly.
 * `pq.Values` is only used by internal connection string parsing methods; even if client code was using it there are no public pq methods that accept or return this type.
 * `pq.NewScanner` is also only used by internal connection string parsing code, not exporting it avoids confusion with [the `sql.Scanner` type](http://golang.org/pkg/database/sql/#Scanner).
 * the unused `ErrNotSupported` variable has been removed.

Strictly speaking these are not backwards compatible changes, but I do not believe any client code can be using these methods or variables. Removing them creates a much cleaner godoc that highlights the parts of pq that authors would actually want to use.