Fixes Issue #13.
This also fixes a similar issue where this library is used via browserify/webpack in the browser (or other environments where `window` is defined). Previously, `var S2 = require("s1-geometry").S2;`, would result in `window.S2` being defined but `S2` remaining undefined. This changes the behavior to prefer exporting via `module.exports`, if available, and falling back to `window` (or whatever `this` is in the module's imported scope -- which is usually `window`). Additionally, this keeps the library from polluting the global scope unnecessarily.
