Migrated changes from 'jacket' into master.

Changes :
- Added vertical management to accompany horizontal media queries.
- Renamed `$mq-breakpoints` to `$mq-horizontal-breakpoints`.
- Moved core mixin operations to functions to allow query-chaining with the `$and` parameter.
- Added shortcut mixins and functions for rapid usage.
- Added doc-headings using Sass comments for readability.
- Added `!global` for inheritance of deprecated `$mq-breakpoints` variable.
- Added deprecation warning for `$mq-breakpoints` variable.
- Added `mq-stringify` function; from Team-Sass/SassyLists's `to-string` function.
- Updated README with some Jacket references.
- Updated Bower Configuration to include Jacket as a dependency.
Completed migration from 'jacket' into master.

Changes:
- Added support for `or` operator.
- Added global variable to control default media type.
- Moved `media type` from the `mixin` to `function` to be able to chain it along the comma-separated list of queries.
- Added function to compute value for breakpoint, used by `mq` function and rasterization output.
- Fixed mixed up parameters.
Updated tests to reflect changes to horizontal breakpoints.

Changes:
- Added test for query-chaining.

Todo:
- Vertical breakpoints to come.
Moved @todos to top of document with related variables.
Improvements to `mq-add-breakpoint` and tests.

Changes:
- Split `mq-add-breakpoint` function to manage horizontal and vertical breakpoints.
- Added vertical breakpoints and updated breakpoint addition.
Updated README

Changes:
- Added explanation about this fork's new features.
- Updated Markdown syntax for variants list.
- Updated list of arguments for the mixin and function.
- Updated code sample for breakpoint lists.
- Updated code sample for breakpoint addition function.