Support skipping database modifications

Introduces new optional constructor parameter $skipDatabaseModifications which defines whether we should skip any attempts to modify the existing database state (includes creating the table and adding / dropping fields based on data passed to constructor). Defaults to false.
Explained in #42

Also contains minor code formatting fixes & composer requirements.

Bug fixes

Merge pull request #1 from kovenant/kovenant-patch-1

Added primary key and some bug fixes

Merge pull request #18 from kermorgant/kermorgant-patch-1

id column handling (avoid removal)

PDO Inheritance support

Merge pull request #5 from stefandoorn/master

Allow $pdo to be nullable, to be able to extend class yourself + adjust VARCHAR into TEXT

PDO Compatibility and Bug Fixes

Closes #2, where the wrong fields where removed from the database table.