(Test) Add solidity-coverage

[fvictorio]

This adds solidity-coverage to get a coverage report of the smart contract tests.

This is a first step. There are some hacky things done to make the tool work (it's still in an early stage of development), and there are some pending things to do:

1. Use the version of truffle that is in node_modules (right now it assumes that truffle is globally installed). This will be necessary for running it in the CI server.
2. Add the coverage report to the coveralls script in the root directory.

[coveralls]

Pull Request Test Coverage Report for Build 40

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 7.513%

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Totals
Change from base Build 37:	0.0%
Covered Lines:	62
Relevant Lines:	739

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💛 - Coveralls