Prevent move into itself

[manidlou]

Fixed the issue with move() when dest is a subdirectory of src. Also, removed shouldMkdirp option as we discussed before.

To quickly remind the issue, with existing behavior, running something like this

const src = 'src'
const dest = 'src/dest'
fs.move(src, dest, err => {
  if (err) return console.log(err)
  console.log('done.')
})

results in just removing src and essentially losing all src contents.

I believe we should prevent this from happening. Any feedback appreciated!

[coveralls]

Coverage increased (+0.07%) to 86.992% when pulling d99d8a1 on prevent-move-into-itself into 85f531e on master.

[jprichardson]

Stellar work @manidlou! @RyanZim please publish a patch when you're able.