Allow promises to be returned from mutations

[rogerballard]

closes #173

[coveralls]

Pull Request Test Coverage Report for Build 752686143

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 96.907%

---

Totals
Change from base Build 538548209:	-0.2%
Covered Lines:	241
Relevant Lines:	243

---

💛 - Coveralls

[kiurchv]

Would it be possible to drop a new release with this functionality?

[kiurchv]

Also, it will be great if this change will be applied to next branch too