Fixing pagination on windows

*This is a duplicate of https://github.com/mojombo/jekyll/pull/1058 which was in a funky state... it didn't seem to recall what branch I made the push from, so I'm recreating it*

**Problem**
On windows the File.basename function will separate paths on `:` so `File.basename('path:num') == 'path'`.

The result currently is that on >= 1.0.0 (on windows) pagination just doesn't work. We end up with the last page being rendered to `_site/page/index.html` and no other page folders at all. This commit fixes that and my pagination on my site behaves like it did in 0.12.1.

**Code Change**
This fix substitutes `:num` for the page number first and calls File.basename after so that `:` is no longer in the path.

**Tests**
The test provided doesn't break on master for linux and mac but does on windows.

Right now my `rake test` is clean on my mac for this branch but dirty for other reasons on windows. I confirmed that it's not more broken than master however.

On my mac and windows machines `rake feature` has many failing tests but on both machines no more failures than on master. I haven't confirmed exactly why to find out if my mac is setup wrong. I'm assuming the breaks on windows are normal non-unix type of things.