Implement FutureJs.toPromise

[seprich]

Even if conversion from (js)Promise to Future is often sufficient on the assumption that all following computation is done within the Future monad, it is still sometimes necessary to convert Future back to Promise. E.g. When writing aws lambdas the ultimate return value must be a promise.

[coveralls]

Pull Request Test Coverage Report for Build 38

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.5%) to 86.131%

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Totals
Change from base Build 34:	1.5%
Covered Lines:	107
Relevant Lines:	121

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💛 - Coveralls

[gilbert]

This looks great. Can you do a quick rebase? I think there's a small redundancy in the test file now.

[MoOx]

Any plan to ship this in a release? =)

[gilbert]

Done!