Added option to make indexes paths more pretty.

Hi,
this PR implements parts of the feature described in #1548. If ```PRETTY_URLS``` and ```INDEXES_PRETTY_PAGE_URL``` are both set to ```True```, index pages get not named ```index-n.html```, but ```page/m/index.html```.

Missing: in case ```INDEXES_PAGES_MAIN``` is set to ```True```, redirects for ```page/1/index.html``` (resp. ```page/n/index.html```, depending on ```INDEXES_STATIC``` config) should be created to the main index page.

Cheers,
Felix

fixes #1548.
Sorry, read your last comment only now. What about having ```INDEXES_PRETTY_PAGE_URL``` either the value ```False```/```None``` (for: don't do it), or ```['page', '{0}', '{1}']``` etc. to specify how precisely the URLs should look?
If we also allow a dict of lists (for making it translatable), we still need to pass ```lang``` through.