[Security] Use cryptographic-quality PRNG

Previous versions of Lasaña used the default Python PRNG which
implements a Mersenne Twister. Although this gave numbers quickly and
in a portable manner, they didn't offer enough strength to be really
unpredictable.

An attacker could exploit this uploading many small files and deducing
the state of the PRNG from the obtained URLs. The attacker could infer
thereafter the URLs that following uploads from other users would get.

Instead, starting in this version Lasaña uses sys.urandom in order to
generate meal id numbers, which in turn requests cryptographic-quality
numbers to the underlying operating system.