Change MouseHandler callback interface to (widget, event)

This is needed to properly support calls to gtk_menu_popup without
forcing the user to by-pass the whole Canvas MouseHandler API.
Add basic support for menus
Add callback to menu item in tests
Merge pull request #21 from timholy/menu

Basic support for menus
fix some formatting in the README
Improve text of README section on Extending Gtk's functionality
Use shorter names for constants, expand exports of constants
Remove _MASK from constant names
fix some GC issues with multiple refs, and changes in Julia base
Add GtkAdjustment, closes #25
Merge pull request #24 from timholy/imageview

Use shorter names for constants, expand exports of constants
Let GtkCanvas objects be double-buffered

This eliminates flicker in ImageView and Winston
Expand usage documentation and add "property browser"
Fix and test signal handler status changes
split out gobject.jl from gtktypes
Merge pull request #32 from timholy/docs

Expand usage documentation and add "property browser"
split out signals.jl
reimplement @gtype using g_type_get_parent
split namespaces (TODO: cleanup export/import)
try detection of gobject type at runtime