Fixes some failing replacements of variable declarations inside scripts

On specifying a `baseUrl` variable inside Selenium Builder the JSON script will contain an entry like this:
```
...
"data": {
    "config": {
        "manual": {
            "baseUrl": "http://some.host.com/page/to/test.html"
        }
    },
    "source": "manual"
},
...
```
However, having a get-step which uses the previously defined `baseUrl`, SeInterpreter wont replace a declaration like `${baseUrl}/login` with the value defined for `${baseUrl}`. The provided pull request tries to solve this issue.

Furthermore, the datasource factory does not allow providing custom datasources which aren't located in `com.sebuilder.interpreter.datasource` package. The pull request sets this package as default package and allows for user-defined custom-packages to look for datasource classes. If no class could be found within the user-defined package if will fall back to the default package.