paginator should validate page and provide default

- use the standard paginator.validate_number method rather
  strict_postive_int.
- support optional paginator method, default_page_number, to get the default
  page number rather than hard-coding it to 1
- this allows supporting non-integer based pagination which can be an
  important performance tweak on extermely large datasets or high request
  loads
- relatively thorough unit tests of the changes
retract the default page stuff.

better way comming in a seperate pr
remove stray func from test
Merge pull request #1187 from ross/paginator_def_val

paginator should validate page and provide default
Added @ross for work on #1187. Thanks!
Explained a bit more about django-filter implementation.

Well, I spent some time trying to gues how djang-filter works, and
if this changes would be introduced, I would have saved this time.
Merge remote-tracking branch 'origin/master'
Small documentation fix
Merge pull request #1191 from jbzdak/master

Filter documentation explanation
Added @jbzdak, for the nice docs improvements in #1191. Thanks!