Integrate Kraken as an external tool

For now two executable files should be added: "kraken" and "kraken-build". On the "External Tools" page in the Application Settings they should be in a common group called "Kraken".

Add these tools to the Linux and Mac OS X external tools packages (the tools are not available on Windows).

Description of the group:

Kraken is a taxonomic sequence classifier that assigns taxonomic labels to short DNA reads.

Description of the "kraken" executable:

The "kraken" executable is used to classify a set of sequences. It does this by examining the k-mers within a read and querying a database with those k-mers.

Description of the "kraken-build" executable:

The "kraken-build" executable is used to build a Kraken database.