Refactor jasmine.Fixtures.createContainer_ to be more concise

jQuery.fn.html() can optionally take a node or string. This means there is no need to use string concatenation when building the container
Fixture.set() & setFixture() return the fixture container
Merge remote-tracking branch 'rupurt/return-container'

* rupurt/return-container:
  Fixture.set() & setFixture() return the fixture container
  Refactor jasmine.Fixtures.createContainer_ to be more concise
v1.5.8