v1: get node_id automatically

[shivdhar]

v1: Create node_id automatically from the MAC address of the machine, with fallback, according to the rules laid out by RFC 4122.

Advantage:

The user can now call now_v1_auto() and get a V1 UUID without needing to figure out how to manually retrieve/create the node_id.

Performance: Comparable

=> cargo +nightly bench --features=v1_auto -- bench_v1
running 2 tests
test bench_v1      ... bench:          65 ns/iter (+/- 15)
test bench_v1_auto ... bench:          66 ns/iter (+/- 6)