Do not force convert Postgres's table name and field name to lower ca…

…se. Fixes #119, #192.

To use lower case table name, do something like this: dbmap.AddTableWithName(Post{}, "posts")
To use lower case field name, specify the field name in type definition: type Post struct { Title   string `db:"title"` }

dialect_postgres.go

@@ -123,7 +123,7 @@ func (d PostgresDialect) InsertAutoIncrToTarget(exec SqlExecutor, insertSql stri
 }
 
 func (d PostgresDialect) QuoteField(f string) string {
-	return `"` + strings.ToLower(f) + `"`
+	return `"` + f + `"`
 }
 
 func (d PostgresDialect) QuotedTableForQuery(schema string, table string) string {