Make appengine func take context.Context rather than an *http.Request
Make appengine func take context.Context rather than an *http.Request

The convention for Google App Engine applications is to pass around a `context.Context` rather than an `*http.Request`. Tyically you pass context.Context around your application and use it to call various GAE services. This update means you can call Colly deep in your code somewhere where you only have `context.Context` available as you'd typically only have `*http.Request` available near the ingress point.

gocolly/colly,gocolly/colly,asciimoo/colly