add request body buffering / parsing helpers

[tj]

in libs but add a few methods here, so common

[jonathanong]

i made raw-body for getting the raw request body with byte limits and content-length checking. i was thinking of putting it in connect but i don't think there's a need.

its implementation in koa-body-parser is pretty simple. you think this is generic enough for core?

var buffer = yield this.rawBody;
var text = buffer.toString('utf8');

[tj]

probably worth leaving out for now