Moving tool_dependencies.xml into a separate BLAST+ package
Introduce package_blast_plus_2_2_26 as dependency

Suggested by Nicola Soranzo, implemented with advice from
Bjoern Gruening. This closes issue #7, see also:
http://lists.bx.psu.edu/pipermail/galaxy-dev/2013-August/015890.html
Update README regarding package_blast_plus_2_2_26 dependency