Updates for testing on Mac OS X.

Mime types on Mac OS X seems to return a slightly different type
for icon image files, so add that to the acceptable list.
Add support for cooccurrence analysis.

This is a relaxed version of collocation analysis. Where
collocations need to be immediate neighbors, cooccurrences can
occur within a given distance of one another, configurable by
setting a window parameter.
Merge branch 'bootstrap' of github.com:rletters/rletters into origin/bootstrap
Datasets, workflows, job views to Bootstrap.
Some more theming.
Switch from padding to margin.
Fix the couple specs that now fail.
Update features and views for specs and features to pass.