Consolidated TraitType Defaults Into One Generator

[rmorshea]

Closes #344

• New method TraitType.default which acts exactly like the methods defined by @default.
  • Due to Create All Default Values With Generators #332, all default values are "dynamic" in the sense that they are made upon request. I created a new method that mirrors the name scheme of @default to help indicate this.
• Fixes a test case inheriting from HasTraits instead of TestCase, which masked Default value of Union is not respected #344.
• New test for Union defaults defined by "subtraits".

Note

TraitType.default could be built into make_dynamic_default by extending its signature, but I'm slightly averse to this idea simply for the sake of uniformity between TraitType.default and @default, and the fact that the name make_dynamic_default is now redundant. If we choose to prefer default to make_dynamic_default, then the functionality of make_dynamic_default will be redundant too and would be on a track towards deprecation.