Introduces DbContext to allow migration to Doobie-RC4

[gaelrenoux]

See #47 for issue and discussion. I've introduced a DbContext parameter when creating the Database layer, allowing to define a LogHandler for Doobie.

I could have passed the DbContext on every call to transaction or autoCommit, but I chose to define it at the layer level. Two main reasons:

1. From a usage perspective, it seems more practical to me to define the log-level of your DB layer when defining said layer, instead of defining it every singe time you run the transaction. Sure, this limits a little bit your options, as you cannot customize the LogHandler on every transaction call, but you can handle that by having multiple Database layers (with different logging levels) and using the one you want.

2. From a long-term perspective: my current implementation for Doobie can be improved. I'm recreating a Transactor every single time a transaction is run, passing it a single JDBC connection instead of a connections source. This allow me to easily control the transaction from the outside, but it might have performance implications. In the future, I'd like to have a single Transactor for the database layer, where I can control which connection it gets by manipulating the source instead. This would not be compatible with passing a LogHandler on each call to transaction.