Pad generated ids to full width

[simonratner]

If generated id can be represented with fewer than 56 bits, the returned buffer and corresponding string representation is shorter than 64 bits. This makes ids non-sortable.

Test case:

var simpleflake = require('simpleflake')
simpleflake(simpleflake.options.epoch + 1, 1)  // <Buffer 80 00 01>