Make cbook safe to import while removing duplicate is_string_like;
closes #1019, replaces #1662.
Update lib/matplotlib/cbook.py

Simplify module docstring, removing mention of possible future changes.
Fix compiler warnings
add EventCollection class to collections
add collections test file with EventCollection tests
add EventCollection test images
add EventCollection demo
add test_collections to standard test suite
add eventplot method to axes class
add eventplot to pyplot via boilerplate
add eventplot test to axes tests
add eventplot test images
add eventplot demo
add EventCollection and eventplot to CHANGELOG
add EventCollection and eventplot to whats_new
small fixes recommended by reviews