Adding MIT license file, keeping consistent with PromiseKit

[lstomberg]

Fixing #6 by adding a copy of the LICENSE from mxcl/PromiseKit. The only modification is the copyright year is set to '2018-present' since this file wasn't added until now. Let me know if this should be consistent with PromiseKit and instead say 2016-present.

[codecov-io]

Codecov Report

Merging #7 into master will decrease coverage by 4.5%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master      #7     +/-   ##
========================================
- Coverage    53.1%   48.6%   -4.4%     
========================================
  Files           8      10      +2     
  Lines         313     278     -35     
  Branches       19       0     -19     
========================================
- Hits          166     135     -31     
+ Misses        146     143      -3     
+ Partials        1       0      -1

Impacted Files	Coverage Δ
Sources/NSURLSession+AnyPromise.m	67.7% <0%> (-25.4%)	⬇️
Sources/NSURLSession+Promise.swift	25.5% <0%> (-2%)	⬇️
Sources/NSNotificationCenter+Promise.swift	100% <0%> (ø)	⬆️
Sources/NSObject+Promise.swift	100% <0%> (ø)	⬆️
Sources/afterlife.swift	100% <0%> (ø)	⬆️
Sources/NSURLSession+AnyPromise.h	100% <0%> (ø)
Sources/NSTask+AnyPromise.h	0% <0%> (ø)
Sources/Process+Promise.swift	45.9% <0%> (+2.7%)	⬆️

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[mxcl]

Many thanks, I appreciate you doing this, I can only do so much multitasking day to day. Will push a version bump.