Dropdown pips can now be centered

- rewrote miss[Right/Left/Top] calculations to work better with
  different dropdown alignments
- created jasmine tests for miss[Right/Left/Top]
- added option `pip: center` to dropdown settings
- created jasmine tests for new pip centering feature

Created Jasmine tests for new dropdown features

Tests for correct miss[Right/Left/Top] behavior.  Dropdowns should
be placed within the outermost row element if one exists.

Tests that 'pip:center' moves the pip to the center of the button
and also tests that dropdowns are centered under buttons that are
larger than the dropdown.

Added vim swap files to gitignore

Added center option for pips

Normal dropdowns with left/right/top/bottom alignments now all can
have pips at the center of the button.

Not yet working for buttons wider than the dropdown.

Added pip alignment section to docs

A section on the `pip: center` option has been added to the "Advanced"
section of the Dropdowns page below "Directions"

Dropdowns working for large buttons

If `pip: center` is set on a button that is wider than the dropdown,
the entire dropdown will be centered along with the pip.

Reordered directions for better testing

fixed missRight calculation

Reorganized dropdown docs page

Fixed pip position on small screens

Pips (and dropdowns) are now correctly positioned on small screens.
- fix pip placement for buttons with `offset().left > 0`
- fix dropdown placement for buttons with `expand` class

Also moved horizontal placement of pip by 3px for centered pips on
top and bottom alignments after measuring with an image editor

Fixed missRight position on large screens