Write proper mocha testsuite for NodeJS

[radare]

And ensure the following tests work as expected

There are some code constructions that can result in failed attempts, when the code looks good.

• openSync() with 1 parameter

var r2pipe = require ('r2pipe');
r2pipe.openSync(function(r2) {
   console.log(r2.cmd('?e hello world'));
});
// TypeError: First argument must be file descriptor

• openSync() with no parameters

var r2pipe = require ('r2pipe');
var r2 = r2pipe.openSync();
console.log(r2.cmd('?e hello world'));
// TypeError: First argument must be file descriptor

• open() with 1 parameter

var r2pipe = require ('r2pipe');
r2pipe.open(function(r2) {
   r2.cmd('?e hello world', console.log);
});
// TypeError: First argument must be file descriptor

• open() with no parameters

var r2pipe = require ('r2pipe');
var r2 = r2pipe.open(); // XXX if no parameters we should connect in sync but continue async
r2.cmd('?e hello world', console.log);
// TypeError: First argument must be file descriptor