Removed test binary from Cargo
Renamed HTTP methods
Ignoring Cargo artefacts
Fixed warnings in original tests
WIP: processing downloads

* write callback gets a closure instead of dumb object as user data. Failed to pass ptr to trait. Probably should get back later
* cleaning up ptrs to closure immediately after processing
Dropped errors enum, added constants

Error code + message should be enough, there is no need to hide curl deep behind as this is actually only wrapper, not a general http engine