feat(middleware) koa support

Closes #6
fix(middleware): Set problem of v8 in the old version of array-uniq

Relate sindresorhus/array-uniq#4 joyent/node#8449
fix(koa) export koa in index.js

It also can be tested in node 0.10
Merge pull request #9 from spmjs/koa

Koa support