Load solc module in lazy fashion. Saves up to 2s on tests.

[zie1ony]

It is handy while working with unit tests.

[tcoulter]

This is good. Thanks for the input.

Before we can commit it, we'll need to move the clean up after solc bit (see the original comments in those files) to happen right after the require('solc') otherwise solc's going to rethrow every javascript error prepending error messages with a ton of minified JavaScript (this makes development a nightmare).

CC @gnidan