Enhance support for custom types

* Add per connection oid to name map
* Global default type format is now based on names not oids
* Add better docs for custom types

doc.go

@@ -0,0 +1,33 @@
+// Package pgx is a PostgreSQL database driver.
+/*
+pgx provides lower level access to PostgreSQL than the standard database/sql
+It remains as similar to the database/sql interface as possible while
+providing better speed and access to PostgreSQL specific features. Import
+github.com/jack/pgx/stdlib to use pgx as a database/sql compatible driver.
+
+Custom Type Support
+
+pgx includes support for the common data types like integers, floats, strings,
+dates, and times that have direct mappings between Go and SQL. Support can be
+added for additional types like point, hstore, numeric, etc. that do not have
+direct mappings in Go by the types implementing Scanner and Encoder.
+
+Custom types can support text or binary formats. Binary format can provide a
+large performance increase. The natural place for deciding the format for a
+value would be in Scanner as it is responsible for decoding the returned data.
+However, that is impossible as the query has already been sent by the time the
+Scanner is invoked. The solution to this is the global DefaultTypeFormats. If a
+custom type prefers binary format it should register it there.
+
+    pgx.DefaultTypeFormats["point"] = pgx.BinaryFormatCode
+
+Note that the type is referred to by name, not by OID. This is because custom
+PostgreSQL types like hstore will have different OIDs on different servers. When
+pgx establishes a connection it queries the pg_type table for all types. It then
+matches the names in DefaultTypeFormats with the returned OIDs and stores it in
+Conn.PgTypes.
+
+See example_custom_type_test.go for an example of a custom type for the
+PostgreSQL point type.
+*/
+package pgx