PostgreSQL and MySQL support

Hi Felipe,

Following https://github.com/meteficha/esqueleto/issues/26, I added flags and #CPP support for the tests, for PostgreSQL and MySQL/MariaDB.

This of course has increased the 'ugliness factor', as those #ifdefs always do, but on the other hand it also makes explicit the different behaviours among databases, which I believe is a net positive.

In this commit I've only changed the tests, not the source, thus maintaining the SQLite pass all status, the PostgreSQL failing two status, and uncovered some additional MySQL failing tests, which I will address in separate issues.

Some MySQL tests also failed by lack of 'specified ordering', but I took a different (more generic) approach this time, with the ```sameElementsAs``` function. Again, this does not alter the SQLite (or PostgresSQL) tests at all.