Fix problems related to streamplot in boilerplate.py
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 evenplot test to axes tests
add eventplot test images
add eventplot demo
add EventCollection and evenplot to CHANGELOG
added EventCollection and eventplot to whats_new
small fixes recommended by reviews
fix bug in agg backend snapping