Use a number formatter to convert strings to numbers instead of asking for the string's doubleValue.
Add mechanism to support formatting/editing of arbitrary object types in the view hierarchy debugger.

Users can register their own formatters for types that PonyDebugger doesn't know about.
Add formatter for UIColor. Currently only supports a hexadecimal ARGB format.
Add "backgroundColor" to the example's view key paths to demonstrate UIColor editing.
Add font formatter for inspecting/changing typefaces and point sizes.
Add "font" property to the example view key paths to demonstrate font editing.
Fix view hierarchy debugging with complex key paths (multiple components).
Add an NSNumberFormatter and an NSDateFormatter to the default formatters.
Merge branch 'viewDebuggingObjectEditing'