Add solhint to lint contracts' code

[unjapones]

Closes #72

[coveralls]

Pull Request Test Coverage Report for Build 214

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 41.727%

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Totals
Change from base Build 211:	0.0%
Covered Lines:	252
Relevant Lines:	553

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💛 - Coveralls