make xlwt real optional

you can install it by `pip install django-xadmin[Excel]`
Merge pull request #49 from flisky/xlwt-optional

make xlwt real optional
add a way to disable queryset.select_related in ListAdminView

it's unacceptable slow when sql inner joins multiple tables
also, only select_related the displaying fields to boost performance
Merge pull request #52 from flisky/select_related

add a way to disable queryset.select_related in ListAdminView