use a covariant generic for Clumps
Merge pull request #20 from fwbrasil/covariant

use a covariant generic for Clumps
Consistentize naming conventions across Clump, ClumpSource and ClumpFetcher
Merge pull request #22 from fwbrasil/consistent-naming

Consistentize naming conventions across Clump, ClumpSource and ClumpFetcher
Clump.list