Create 'flipValueOf' module

Move the map of properties that require their values to be flipped, and
the value-flipping function, into their own private module.

The module includes normalization of vendor-prefixed properties,
ensuring that their values will also be flipped.
Support 'transition' and 'transition-property'

Support property-flipping in the values of `transition` and
`transition-property`.

Fix gh-8
Minor reorder of index.js
Rename test file
Minor tidy up; update HISTORY