Esprima Rewrite

**This isn't ready for merge**, but I wanted to go ahead and get the PR up for feedback.

This PR:

* Replaces the current build DSL with ES6 template strings
* Removes CoffeeScript support (run on compiled CS output instead!)
* Uses Esprima for parsing, making support for multi-line strings, etc. more robust
* Uses the source replacer from [es6-module-loader](https://github.com/ModuleLoader/es6-module-loader) for rewriting
* Currently fixes:
  * https://github.com/square/es6-module-transpiler/issues/25
  * https://github.com/square/es6-module-transpiler/issues/26
  * https://github.com/square/es6-module-transpiler/issues/27
  * https://github.com/square/es6-module-transpiler/issues/28
  * https://github.com/square/es6-module-transpiler/issues/31
  * https://github.com/square/es6-module-transpiler/issues/32
  * https://github.com/square/es6-module-transpiler/issues/33
  * https://github.com/square/es6-module-transpiler/issues/34
  * https://github.com/square/es6-module-transpiler/issues/24
  * https://github.com/square/es6-module-transpiler/issues/11
  * #46 

Outstanding issues:

* Weird issues with accidentally dropping newlines when rewriting code that lacked semicolons
* Docs need new information (talk about the new `__default__`)