cmd/tsuru-base: introduce jsonWriter

This writer gets a slice of bytes, unmarshalls it as a list of logs, and
write them in an internal writer.
cmd, cmd/crane, cmd/tsuru{-base}: fix SetUpSuite

Ops! x)
cmd/tsuru-base: implement --follow feature in the client

One of the most wanted features :-)

Closes #194.
cmd/tsuru: version 0.7.1
cmd/crane: version 0.4.1