Fixing Contract Development

[ngotchac]

The contract development tool was in fact broken due to a bug in Chrome (seems to be fixed in v56).

This PR adds a fallback to Solidity compilation in the main thread (thus making the whole UI to freeze when compiling). It also removes the Worker which is replaced by a service worker. This will be used if possible, and will at least be used to cache Solidity versions so users don't need to be online once fetched once.

[coveralls]

Changes Unknown when pulling efb928b on ng-contract-dev into ** on master**.

[jacogr]

Compilation failure on CI -

ERROR in ./serviceWorker.js
    Module not found: Error: Can't resolve 'promise-worker/register' in '/builds/Mirrors/ethcore-parity/js/src'
     @ ./serviceWorker.js 17:0-60

[jacogr]

Looks good. Bit worried that we are not able to test on FF at this point to verify the other side (due to the connection issues). Chrome "freeze" is irritating, but it is what it is.

[coveralls]

Changes Unknown when pulling 7b3b82b on ng-contract-dev into ** on master**.