feat(dematerialize): add dematerialize operator

[kwonoj]

• add dematerialize operator
• add marble test case for materialize, dematerialize operator

closes #391, #475

Effort to try close above 2 opened issues. Migrated RxJS4 tests for materialize operator, added coverage for dematerialize as well.

[benlesh]

@kwonoj, can you break this into two commits? I tried to do it myself, but I can't do it without putting the commits under my name.

basically it's just git reset HEAD~, then git add the files individually before your commit.

The two commits I'd like are:

1. feat(dematerialize): add demat... (your text here)
2. test(materialize): add tests for ... (your text here)

.. be sure to include the issue numbers they close.

Again, I'm happy to do this for you, I just don't want you to lose your github "cred". ;)

[kwonoj]

Sure, I'll give it a try and update PR.

[kwonoj]

Update PR to contain 2 separated commits for adding dematerialize operator and expanding test coverage for materialize.

[benlesh]

Merged with ff1a7f9! Thanks, as always @kwonoj. :)