chore(coverage): generate coverage report on dev-server when COV=true
chore: ignore some stuff for coverage
test(cachier): improve coverage for cachier
doc(README): add about coverage mode
test(cachier): fix .connection spec on Firefox

Seems that the IDBDatabase constructor, on Firefox, has no name. Still, we can
use toString() to introspect the returned object from the promise.
Merge pull request #52 from bemusic/dev-coverage

Enable code coverage on dev-server