Build fetcher-core

[nachog00]

As explained in #30 , we need to refactor fetcher into one (maybe two) new crate.

fetcher-core

This crate must accomplish the following:

• Take in a path as input form its consumer
• Manage fetched resources in the above mentioned directory
• Implement unit testing
• Implement public API that allows
  • the consumer to request specific resources
  • those specific resources to be cached in runtime if not present already