Dropdown pips can now be centered

Issue #6026 contained a request for an option to center the pips on dropdowns.  This pull request implements that feature.

While working on centering pips, I rewrote most of Arthur's (arthur at zurb dot com) miss[Right/Left/Top] code.  There are now Jasmine tests for correct behavior in this case, but someone from Zurb should probably review this to make sure I interpreted the intent of the original code correctly.

- 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