Use the same unicode aliases as Safari 7.0.5 on OS X 10.9.4

Category-Emoji.json contains many code point sequences ending in U+FE0F
VARIATION SELECTOR-16. OS X renders some of these code point sequences
as color emoji even if you strip off VARIATION SELECTOR-16. (And in fact
it even renders some of them as color emoji if you replace VARIATION
SELECTOR-16 with VARIATION SELECTOR-15, even though that is explicitly
requesting a "text" variant of the character).

We used to assume that VARIATION SELECTOR-16 was optional for all emoji.
But this doesn't match OS X's behavior. This could cause problems, e.g.,
if you were to use the unicode_aliases property to detect Unicode emoji
and replace them with images for browsers that don't support emoji
natively.

The Ruby code no longer does anything special with VARIATION SELECTOR-16
other than to remove it from the description of each emoji. The new
db/aliases.html file can be used to determine which the correct Unicode
aliases for each emoji are. That page takes each emoji, generates a set
of variants using VARIATION SELECTOR-16 and VARIATION SELECTOR-15,
and renders each variant into a <canvas> element twice: once with a red
font color, and once with a green font color. If the variant renders the
same way, the OS is rendering the variant as a color emoji, and we add
it to the "unicodes" array for that emoji.

So the new process for generating db/emoji.json is:

1. rake db:dump > tmp
2. mv tmp db/emoji.json
3. open -a Safari db/aliases.html
4. Copy the resulting JSON from Safari
5. pbpaste > db/emoji.json

This ensures that unicode_aliases only contains those code point
sequences which actually render as color emoji on OS X.
Remove incorrect unicode aliases

db/aliases.html now clears out any incorrect aliases it finds.
Fix tests
Merge pull request #56 from github/match-osx-behavior

Use the same unicode aliases as Safari 7.0.5 on OS X 10.9.4