Share value of inherited sequences with parent factory

This addresses https://github.com/bkeepers/rosie/issues/24
BUGFIX: pass options to the after callback as per README
clone attributes

This prevents the attributes from being mutated during the build
process, if they have been specified.
Add tests to ensure sequences work properly
Merge pull request #25 from TabDigital/master

Share value of inherited sequences with parent factory
Merge pull request #27 from RustyToms/sequences-with-specified-attributes

Fix sequences with specified attributes
Extract `extend` and `hasOwnProp` utilities out.

`extend` uses `hasOwnProp`, and `hasOwnProp` is there mostly for safety. You cannot always assume it will be present on a random object, so it's better to use it as shown in this commit.
v0.3.1
Replace a bunch of "nullable" doc comments with "optional".
README: add syntax highlighting

just some visual sugar :)
Merge pull request #29 from revolunet/patch-1

README: add syntax highlighting