Allow dashed schemas

[guzman-raphael]

This fix required introducing containers.Map for indexing into schemas. This means a breaking change.

For example:

Let's assume that connection is an instance of dj.Connection.

Before you could access a schema like the below but they are no longer supported.

connection.schemas.example_schema
connection.schemas.('example_schema')

Now to index into schemas you would perform.

connection.schemas('example_schema')