Add support for integer, float and text arrays

Restructure internals a bit so pgx/stdlib can turn off binary encoding and
receive text back for array types.

README.md

@@ -70,9 +70,6 @@ if err != nil {
 }
 ```
 
-Prepared statements will use the binary transmission when possible. This can
-substantially increase performance.
-
 ### Explicit Connection Pool
 
 Connection pool usage is explicit and configurable. In pgx, a connection can
@@ -151,9 +148,17 @@ point type.
 pgx includes Null* types in a similar fashion to database/sql that implement the
 necessary interfaces to be encoded and scanned.
 
+### Array Mapping
+
+pgx maps between int16, int32, int64, float32, float64, and string Go slices
+and the equivalent PostgreSQL array type. Go slices of native types do not
+support nulls, so if a PostgreSQL array that contains a slice is read into a
+native Go slice an error will occur.
+
 ### Logging
 
-pgx connections optionally accept a logger from the [log15 package](http://gopkg.in/inconshreveable/log15.v2).
+pgx connections optionally accept a logger from the [log15
+package](http://gopkg.in/inconshreveable/log15.v2).
 
 ## Testing