v.0.5.6

This version contains a major change to add support for nested transactions. It is not a breaking change, that's why only minor version increment was done, even though supporting nested transactions represents a major milestone for the library.

Chapter Nested Transactions was added to documentation with a code example.

Things still outstanding as far as the full PG support goes:

• Support for SAVEPOINT - not sure how this can fit into a promise chain at all, and who really needs it;
• Support for cursors - so few developers really use those, because they are mostly employed within function implementation, as a proper approach, not on the client side.