Add. Support multiple database backends.

```Lua
local Database = require 'resources.functions.database'

-- Default backend configured via xml_handler.db_backend = 'native'
dbh = Database.new("system")

-- To use other backends you can use
dbh = Database.backend.luasql("system")
```

resources/install/scripts/self_test.lua

@@ -0,0 +1,12 @@
+local Cache    = require 'resources.functions.cache'
+local Database = require 'resources.functions.database'
+
+Database.__self_test__({
+  "native",
+  "luasql",
+  "odbc",
+  "odbcpool",
+},
+"system")
+
+Cache._self_test()