Adding utility for keeping database schemas in memory.
PostgreSQL: Adding support for upper.io/db/util/schema.
Adding HasTable() method, to determine if a table has already been added to the in-memory schema.
PostgreSQL: An error was missing after trying to use tx.Collection() after the transaction was completed.
PostgreSQL: golint cleaning.
schema: Adding docs.
Closing #5. PostgreSQL now returns error if the user attempts to use Open() with wrong credentials.
MySQL: Fixing visibility of result{}, table{} and source{}.
MySQL: Adding support for the 'schema' package and removing golint's complaints.
PostgreSQL: Cosmetics.
SQLite3: Adding support for the 'schema' package and removing golint's complaints.