Support long private keys in latest Chrome 32

In latest Chrome 32 I'm getting long private key and signature (> 256 bytes).
It means that writing directly to buffer bytes does not work correct:
 ````js
crx[8] = keyLength
crx[12] = sigLength
````
When trying to install such crx you may get `CRX_ZERO_SIGNATURE_LENGTH` or `CRX_ZERO_KEY_LENGTH` errors.

This pr fixes it via using `buffer.writeUInt32LE` method.
Hope to be merged soon.
Thanks!