Adds note about storing multiple tokens.

It's a bit wordy, welcome to word-smithing suggestions.
Merge pull request #147 from davedash/patch-1

Adds note about storing multiple tokens.
Fix a bug causeing `gist -c` fails for xsel users.

Unlike `xclip`, calling `xsel` without any argument outputs the
selection.
In other words, `xclip` defaults to `xclip -i`,
but xsel` defaults to `xsel -o`.

Thus we should use `xsel -i` instead of `xsel` for copying the url
to clipboard.
Merge branch 'master' of github.com:weakish/gist
bump version
Remove JSON dependency [Fixes #152]