Add first & last in between relationships

[bcodell]

This PR contains a failed test due to a potential bug in the dataset macro's query compilation logic.

Resolves #8 once potential bug is addressed.

This PR:

• adds macros for first_in_between and last_in_between relationships
• adds appropriate tests
• adds .DS_Store to .gitignore
• removes symlink from integration_tests/dbt_packages (was causing version control issues in local dev/testing)
• adds links to source code for new macros in readme