Added support for Solidity 'import' statements

[Antanukas]

• Using solc compile function which calls a callback when import statement gets detected. This function is responsible for loading file source.
• Upgraded solc version to latest just in case.

[uzyn]

@Antanukas Thanks. Can you link to a solidity code that we could test this new feature?

On a separate note, we're going to need unit tests and better management on the loaders soon. (uzyn/ethereum-webpack-example-dapp#3 is a good start)

[Antanukas]

I have updated example-dapp with additional contract in external file which gets imported. Also added dummy library that gets imported. https://github.com/Antanukas/ethereum-webpack-example-dapp

This also tests web3-loader

[graup]

Thanks @Antanukas, just what I needed now! You just saved me a lot of time.

This works for me, let's get it merged!