use the README to explore some behaviour guidlines
add a TODO
make Raca::Container#delete return a boolean to indicate success

* We don't want to return a raw HTTPResponse
make Raca::Container#purge_from_akamai return a boolean to indicate success

* We don't want to return a raw HTTPResponse
make Raca::Container#download return the number of downloaded bytes

* We don't want to return a raw HTTPResponse
add some missing comments to Raca::Container