fixes #104: fixes duplicate fields issue and adds support for compound primary keys

[tomjoro]

fixes #104

• Add support for composite primary keys
• Fixes bug where repeated keys in indexes generate duplicate fields.

Composite primary keys are supported by generated multiple true primary_key fields according to ecto specifications.

I also changed code so it only emits primary_key when it is true. This makes it much more readable.

This fix only affects Postgres.