Add npm init

[et]

If you don't do this, you'll receive this error:

❯ zeppelin npm install zeppelin-solidity
npm WARN saveError ENOENT: no such file or directory, open '/Users/et/package.json'
npm WARN enoent ENOENT: no such file or directory, open '/Users/et/package.json'
npm WARN et No description
npm WARN et No repository field.
npm WARN et No README data
npm WARN et No license field.

+ zeppelin-solidity@1.3.0
updated 1 package in 0.677s

[frangio]

Thanks @et!