Refactor existing caching mechanism into pluggable system

[unjello]

Create an interface Cache, similiar to storage.Storage that will allow for pluggable caching extensions. Caching subsystem is created with full access to Request and Response objects, to potentialy make more complex caching decisions based on both request and response headers. Because of the need to access those objects, Cache is not created in its own package like storage, but in root colly.

Closes #103

[unjello]

@asciimoo would you happen to have a second to review this?

[WGH-]

I certainly approve the efforts do decouple caching from the guts of colly.