Send invoke implementation

[slipo]

This takes care of #13

Uses neon-js api.

Tested with:

  {
    scriptHash: 'b3a14d99a3fb6646c78bf2f4e2f25a7964d2956a',
    operation: 'putvalue',
    arg1: 'test',
    arg2: '1',
    assetType: 'GAS',
    assetAmount: '.00025'
  }

Should show a success message and the transaction id.