support for 'that the value exists' type requirements
added

* no_requirements
* wf_ functions now need to be tagged
updated docs, tagging wf_ functions
removed a priority from the example
possible parallel support, doc/example
full python path for import example
Merge branch 'master' of github.com:qiime/qiime into shortcircuit
removed parallel, added staging function
Merge branch 'shortcircuit' into wf_qual_filter_fasta
added sanity_check
sanity check, staging method
further progress