implement a mock dht for use in testing
change back to using Client method
move mock routing tests to proper directory
Merge branch 'feat/mock-dht'
feat(util:context) add error logging to context

This commit...

is a second attempt at: https://github.com/jbenet/go-ipfs/pull/68

partially addresses: https://github.com/jbenet/go-ipfs/issues/66

is the result of discussion at:

    https://gist.github.com/perfmode/f2951c1ed3a02c484d0b