Make project multi-module

[jdegoes]

We need the following modules:

• core, which contains the core data types
• driver, which contains the ZIO definitions of database operations, independent of a backend
• jdbc, which contains a lowest-common denominator JDBC "driver" for the ZIO definitions, which assumes ANSI-92 compliance
• postgres, which contains a Postgres dialect and driver
• mysql, which contains a Postgres dialect and JDBC-based driver
• sqlserver, which contains a SQL Server dialect and JDBC-based driver
• oracle, which contains an Oracle dialect and JDBC-based driver
• spark, which contains a Spark SQL dialect and Spark-based driver
• cassandra, which contains a Cassandra SQL dialect and Cassandra-based driver
• test, which contains a test "driver" for the ZIO definitions

We can probably start with just core, driver, jdbc, and test.

This ticket involves only creating the SBT project structure and directory structure, not implementing any modules.

[jczuchnowski]

I'll take this one.