Support using custom primary key

[jensjeflensje]

As raised in #8 (by me :p), using a custom primary key is not supported. I've added this functionality, as well as some other changes:

• Separate StormModel into StormModel and BaseStormModel, with StormModel still containing the same things as before. BaseStormModel just doesn't have the primary key (id field)
• Made methods for obtaining the primary key field and value, as this isn't always the id field anymore
• Changed all relevant references of the getId method to getPk

Other changes:

• A where clause is now also using the fields toSql method to convert the value into a usable format for querying. This worked for most datatypes before my PR, but not UUID
• I've added the memory file to the .gitignore as it's a temporary database for tests (right?)
• The save method doesn't return anything anymore, as the returned value was inconsistent in nature before (sometimes the generated key, sometimes the changed rows). I personally don't think it's needed

[Mindgamesnl]

Please see the comment on the accompanying issue for details; #8 (comment).