REDCORE-260 Fix module installation

When redCORE is installed from an extension the mod_redcore_language_switcher is not installed because is not found by the redCORE installer.

This fix uses a feature already provided in the installer https://github.com/redCOMPONENT-COM/redCORE/blob/develop/install.php#L287 setting the right client. This fix solves all current issues when installing redCORE from an extension.

Still this fix is going to be improved with a future pull that will provide a common script for all extensions as agreed with @Kixo. But while that step is done this fix provides a solution backwards compatible with all our extensions.

![screen shot 2014-11-14 at 15 42 50](https://cloud.githubusercontent.com/assets/1375475/5047326/822b3868-6c14-11e4-9604-a62272f00045.png)

I have tested:
- [x] installing redCORE as standalone package: Works! the language switcher module is installed
- [x] installing redSLIDER2 containing redCORE framework: Works! the language switcher module is installed