Merge pull request #2 from Norberg/master

Added dependency to Pyglet 1.2 or higher.
Minor refactor to dialog's delete.
Minor changes to comments, added parent = None in viewer's delete.
Made Wrapper's "content" a property. Dialog is now its own manager.
Added test for substituting widgets inside a dialog.
Dialog's position is now set on window resize. Added test for that.
Removed old attribute from Dialog.
Removed on_escape and on_press interface from Dialog.
Removed redundant method definition.
Removed unused method.
Made offset a property of dialog.
Changed test from "viewer" to "dialog". Added test for offset change.
Moved anchor property to wrapper.
Removed unused import.
Minor fixes in comments.
Removed unused method and improved documentation.
Made dialog's window a property and added test of dialog without window.
Set content of container to be a property.
Fix error on grid's add_row.
Simplified grid initialization. Now content is a required argument.