move hash_local_file into utils
add dependency on requests
add plugin CLI
add plugin exceptions
implement plugins
add tests for plugins
add CLI<->backend glue for plugins
allow cmdline handlers to yield a return code
move download() into utils
use unicode arrows
add tests for plugin cmdline handlers
Python 3: use {}.items() instead of .iteritems()
fix plugin download URLs
raise exception on download error
add missing --force option
show version after plugin installation
fix tests
add plugin docs
add warning about plugin trustworthiness
show plugin help text after install