Version 1.0.2

Actually, this is good enough for the 1.0.0 release

I don't see the api changing anytime soon, so I'm going to call this the
1.0.0.

Bumping the version number because of the performance increase.

After b1ea2e3, the performance of sequencial runs should be about the
same as jekyll without this plugin.

Fixes #1

Releasing the fix for @2x on second build.

Updated the readme to explain this fork.