Would be nice to add support for solcjs

[dylanjw]

What was wrong?

py-solc strictly requires the solc binary and does not work with solcjs. Many people would like to install solc with npm install solc.

Merge changes in https://github.com/Aviah/py-solcjs to this project, with an added feature to detect which version of solc is present on the system

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