prefix all vars with fullstory, add descriptions to README, general hygiene

[huttotw]

A couple things here:

1. Prefixing all of our vars with the standard fullstory_ will make sure that our vars will not collide with any other DBT package.
2. When using this DBT package, I found it necessary to be able to provide the database, schema and table name explicitly so that we don't need to run the consuming DBT project in the same dataset as the raw events table. To support this, I introduced: fullstory_events_database, fullstory_events_schema and fullstory_events_table. This allows us to sync the events table once using Data Destinations, then use the same table across many projects or datasets (Big Query).
3. Updated variable names to be more consistent / better indicate their purpose.

@craigrmccown this also addresses feedback here.

[sgagofs]

LGTM!