v1.3.0

Some big changes, mostly on the dev side:

• use proper build setup based on Rollup, life Leaflet itself
• import leaflet-rotatedmarker as a dependency instead of having duplicate code in both projects
• use Babel transpilation to code in ES6 syntax
• big simplification of very old and ugly bits of code

And at last, something that should have been done years ago, don't compute invisible symbols to avoid big perf drop when zooming in. This required dropping the cache system, but this one was a very ill-based optimization, not bringing that much benefit, and it will be easier to focus on other perf improvements now.