Akiban Driver Implementation that Uses PDO

Based on discussion in pull request 188 with @stof, I decided to modify the implementation for Akiban to use the PostgreSQL PDO driver instead of the native PHP client for PostgreSQL.

One piece of functionality that does not work with current version of Akiban is lastInsertId. This is because the implementation of CURRVAL for Akiban requires both schema name and sequence name be provided whereas PostgreSQL only requires sequence name ([relevant source](https://github.com/php/php-src/blob/master/ext/pdo_pgsql/pgsql_driver.c#L360)). I will attempt to get CURRVAL in Akiban implemented to have the same parameters as PostgreSQL.

This causes a few tests to fail in the test suite right now. Is there a way to mark a test as expected to fail for a specific database driver with the test suite? What I did in this pull request is skip any tests that use lastInsertId when testing against the Akiban platform. I'll update these tests to not be skipped when CURRVAL works as expected. Let me know if there is a better way to do that.