Gebrselassie

UI:

• consume: Allows to specify offsets per partition (cf #3).
• produce: Allows to specify key, value and partition (cf #17).
• topic: Sorts topics alphabetically (cf #13). Thanks @echojc

Internal:

• Use godeps for vendoring (cf #16). Reduces repository's footprint. Thanks @marianogappa
• Updates to README and command usage information displayed for -help.

Breaking changes:

• consume: Drops text output and -json flag, defaults to JSON output (cf #15).