add H2 support

[jhamrsky]

This PR adds H2 DB support and closes #7.

Roughly, following was done:

• Unified SQL syntax by removing "RETURNING" statement, which is specific for PG.
  • Used HugSQL feature of .getGeneratedKeys in combination with custom method.
  • Method names were left with YeSQL syntax using <! in order to somehow mark that the caller should keep in mind returning value.
  • Introduced *db-type* based on JDBC connection string which can be used for SQL-dialect specific purposes.
• Added H2 support and files
  • Migrations are based on PG migrations. Most changes just remove PG specific calls.
  • For SQL compabibility of calls of array_agg, ArrayAggregator was added to code (Java).
  • Extended interface on required classes for reading parameters.
  • Adjusted project/main file to support different migration schema. PG set as default one.
• Added some text to readme about H2.
• Tested functionalities with PG and H2.

If it comes to full text search, H2 support is somewhat limited (no weighting support). I intentionally left out wildcard support (has some performance issues) and used native search engine. But otherwise, H2 is nice for more lightweight use cases.

[yogthos]

Hi,

Thanks for the PR, this looks pretty comprehensive. :)