Don't obtain a Proj4 object if we aren't going to use it (i.e. projecting 4326 <--> 900913).

Performance on `sf_streets.json` from nodetiles-init:
FeatureCollection x 4.38 ops/sec ±3.99% (15 runs sampled)
Feature x 5.38 ops/sec ±3.32% (18 runs sampled)
Old FeatureCollection x 4.27 ops/sec ±4.31% (15 runs sampled)
Old Feature x 0.75 ops/sec ±2.86% (6 runs sampled)
Allow Proj4 instances to be passed into `project.FeatureCollection()`. Also abstract the process of getting a proj4 instance. No major performance impact! Fixes #54.