Get emoji list & aliases from data file instead of symlinks

Previously, emoji name & unicode aliases were determined by following
symlinks among `images/emoji/*.png`. This led to nontrivial code for
resolving these aliases, made it tricky for contributors to add new
aliases and inspect existing ones, and didn't leave room for adding
metadata to emojis such as tags or descriptions from the Unicode spec.

Moreover, the aliases as symlinks led to duplication of image assets in
users' applications, with `hocho.png` and `knife.png` representing the
same emoji but being two separate images. Users were also unsure what to
do with `unicode/{HEX-NAME}.png` files, which would end up among their
images after running the `:emoji` task.

This change removes the symlinks support and creates the list of emojis
and their aliases in `emoji.json`. A single emoji is now represented with
an Emoji::Character instance, which has the `image_filename` method to
determine the path to the corresponding image instead of having to
construct it manually.
Remove duplicate image for shipit squirrel
Add task to download Unicode 6.3 names list

This list contains textual descriptions for emoji characters.
Add dump script to inspect Unicode descriptions & aliases

This script was used to construct the initial `emoji.json` file.

Example:

    rake db:dump | less -r
Improve emoji lookup fallback behavior

Instead of an unfriendly LocalJumpError, raise the Emoji::NotFound
exception with a helpful message if the block was not given.

If a fallback block was given, yield the value for which the lookup failed.
Update usage examples in the README with the new API
Have `unicode_aliases` include emoji's raw representation as well

This brings it up to par with `aliases`, which includes `name` as well.
Tweak tags for some emoji

Ensure that tags accurately describe the original meaning of the emoji
(with respect to its definition from Unicode spec).