Add #to_s and #inspect for classes that include CommonMethods
Use a dedicated object for testing
Add Common::Inspect to more classes
Make #to_s look like (Shoes::Class)

This is actually what Shoes 3 does
Test #inspect on an anonymous class
Update Color's #to_s, #inspect
Change default #inspect format

Was: (Shoes::ClassName <0x01234567>)
Now: (Shoes::ClassName:0x01234567)

Now, by default, Shoes objects inspect like normal Ruby objects, only
with the parentheses from Shoes 3, instead of angle brackets. This
change makes sense since we are now using '::' to delimit namespaces.
Add #inspect for Gradient

Was: <Shoes::Gradient #000000->#ffffff>
Now: (Shoes::Gradient rgba(0, 0, 0, 255)->rgba(255, 255, 255, 255))

I think readability suffers here, but perhaps rgb is actually more
readable. In any case, it's important that the color representations be
consistent, so this matches the current implementation for Color.
Add #inspect for Text, Span, Link

span.to_s    #=> "text"
span.inspect #=> (Shoes::Span:0x01234567 "text")
Add #to_s, #inspect for Point

point.to_s    #=> (40,50)
point.inspect #=> (Shoes::Point:0x01234567 (40,50))
Update #inspect for Dimensions and Point

Was: <Shoes::Dimensions:0x000047e4 relative: (10, 20)->(100, 200), \
      absolute: (, )->(, ) 90x180>

Now: (Shoes::Dimensions:0x000047e4 relative:(10,20)->(100,200) \
      absolute:(_,_)->(_,_) 90x180)

Both Point and Dimensions use '_' rather than the empty string for
values that are falsey. This makes it clear that there's an spot there
with nothing in it, rather than looking like a mistake.
Display colors as rgb(0, 0, 0) (omit alpha)

This makes us compatible with Shoes 3. We show the alpha in #inspect, so
it's available if necessary. Also, pull specs into separate describe
block for inspect, to reduce clutter.
Fix specs for Gradient to match new Color#to_s
Merge pull request #747 from shoes/inspect

Add common #to_s and #inspect methods for DSL objects

Changelog: improvement