Fix polyglossia/biblatex problem with xelatex

When using biblatex, xelatex failed with the following error message:

    ! Undefined control sequence.
    <argument> blx@lng@\bbl@main@language 

This is due to the fact that polyglossia/babel is loaded *after* biblatex. Reverse the order, and the problem is fixed. My suggested solution is to move polyglossia/babel definitions to before biblatex.