all finalized messy svg files for heart
updated heart-12 design and cleaned up all svg files
updated maki.json to include heart, rendered all heart files
Merge pull request #142 from mapbox/heart

Add Heart icon
Merge branch 'gh-pages' into rails
added all 18px svgs and updated all 18px svgs
final clean svg for rail-metro-24, final messy svg files for all others
final clean svgs for all 3 icons at 12, 18, and 24px
deleted old rail icon, renamed rail-heavy as rail
updated maki.json to include rail-light and rail-metro, deprecated rail-above and rail-below, ran render.sh