Updates page numbering and floating header styles

- Adds page numbering to next/previous links
- Updates floating header button styles to make them more minimal
- Updates JS algorithm used to calculate top position of an element,
which fixes an error in Safari that returned incorrect values when the
page was scrolled.
- Moves header summary to a partial
- Encapsulates page variables into a single hash.
Updates result summary helper spec

Updates variable names used in this spec, as they are now wrapped in a
hash.
Adds pagination tests

Adds spec and cassettes for testing pagination functionality on search
results page.
Fixes failing test

Updates test to handled updated return to results button.
Merge pull request #246 from codeforamerica/page-numbers

Page numbers