Add support for database URLs

[mikerockett]

Adds basic support for database URLs, or DSNs. It does so via the addition of a DSN parser that is derived from the one Laravel provides (it's a simplified version that only works with strings).

Resolves #195.

Note: Tests were failing due to the recent addition of quoted usernames in PostgreSql – these have also been fixed. There was also test case in PostgreSqlTest that was seemingly copied over from MySqlTest, in that it included a reference to the credentials text file – this has been corrected as well.

[freekmurze]

Very nice, thank you!