Add listTree

Adds a listTree method that returns an array of file objects for a particular revision. 

Sample response: 

```
[ { sha: '706d95f7efbae5974f1072289bddc107f902345a', name: '.gitignore' },
  { sha: 'cde4728a2cc06102b0c151af1047b96d54404fa4', name: 'LICENSE' },
  { sha: '1bf63affe3f7a3d1894dcc6bdefb732420067db9', name: 'README.md' },
  { sha: '4e98585606c61d1c17f74fcc17c2ea9849954a08', name: 'examples/binary.jpg' },
  { sha: '09d1d79fdccd09a28454377ca5056e1667f65b15', name: 'examples/server.js' },
  { sha: '57fe4249c69ef279379c2be4124dda5014671597', name: 'index.js' },
  { sha: 'ba8fda30990698bc9a02568219b1d3f60fb71ca9', name: 'package.json' } ]
```