Introduce Git tags that correspond to the PostgreSQL version

[niko-kriznik-globtim]

There are API differences between PosgreSQL version.
For example, #4 removes the now obsolete 72000 error.

[jackc]

I'm not sure tagging a version of pgerrcode for each PG version is the best approach. It is quite likely some users of pgerrcode will connect to multiple different versions of PostgreSQL.

I think it would be better to keep the obsolete error codes supported. The easiest way to do this would be to move the obsolete code to another file so the main errcode.go file could continue to be generated.