Refactor tests.

DelocalizableClass was shared between tests - better use a new anonymous class for each test.
Make sure class attributes are not shared between subclasses.

Using `class_attribute` with arrays or hashes is tricky, as mutating
the object will change the object of the superclass (which may be shared
by other subclasses, too).
Add proper development dependencies in gemspec
Delocalize a given :value unless it's a string

See pull request/issue #31 for details.
Merge pull request #50 from die-antwort/fix-class-attribute-sharing

Make sure class attributes are not shared between subclasses.

`class_attribute` doesn't work properly for arrays and hashes so we need to work around this issue by duping the parent class' attributes if necessary.
Merge remote-tracking branch 'clemens/spike' into spike

Conflicts:
	lib/delocalize/delocalizable.rb
	test/delocalize_test.rb