Initial kraken implementation

Ability to fetch trades
Retry method
Added trading capability to the kraken module
Add kraken-api module dependency
Make sure we have trades to process

Prevents a crash with Kraken where we only fetch what's changed
since last request. This will sometimes return an empty resultset
which would cause a crash in the tradeFetcher.
Add kraken in the comment for the exchanges in the config
noteOrder doesn't expect the error (just send txid).
Fix log message when not enough in balance (was mixing up currency/asset).
Merge pull request #181 from iceydee/kraken

Kraken integration (fixes #87)