Allow customizeable database system

[mellowagain]

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Currently, Shiro only supports MySQL. This is not as customizeable as I want it to be.

I propose that we add support for the following two database systems, in addition to current MySQL support:

• SQLite
• PostgreSQL

Thanks to the flexibility of sqlpp, only a few parts (namely connection init) of the code need to be changed up for this to work. There is also the ability to use odbc, which I still need to evaluate if it's of value or not.