schema, docs, and invoice cast updates

[fivetran-joemarkiewicz]

This PR includes the following changes:

• Addition of the custom quickbooks_staging schema as well as relevant README updates reflecting this change.
• Casting fix of the deposit_to_account_id field within the invoice staging model to address Issue #8.
• Inclusion of the docs folder to allow for github pages docs site deployment once the branch is merged with master.
• Package version upgrade from v0.1.2 -> v0.2.0 as the custom schema introduces a breaking change.