Add object models for Schema and Catalog

[mdelaurentis]

Add object models for Schema and Catalog. See https://github.com/singer-io/tap-mysql/pull/15 for example usage.

• Add singer.schema. This contains a Schema class that represents a JSON Schema. If you're building a schema from scratch, you can use the constructor. If you're loading it from a file, you can use Schema.load().
• Add singer.catalog. This contains a Catalog class. Usage is similar to singer.schema.Schema.
• Add a --catalog option to the command-line options defined in singer.utils. singer.utils.parse_args will now load a Catalog object automatically if the --catalog option is defined. I left the --properties option there but marked it as deprecated.

[mdelaurentis]

@bi1yeu my reason for choosing -a as the short option for --catalog was silly. -c is already used for --config. I didn't want to use -C because it's too close to -c. I chose -a because it's the next letter in "catalog". But I don't really like that.

I just removed the short option. We can always add one later. Thanks for bringing that up.