Release 0.2.0

Features

• Cargo.toml now correctly includes both licences (Apache-2.0 OR MIT) in the license field.
• A new CHANGELOG.md file lists (breaking) changes between versions.

Breaking changes

• The default generator implementation was switched to lock-free.
  
  I.e. the blocking feature is now turned off by default, and lock-free is enabled. If your code uses
  Generator::generate_blocking(), you'll have to enable the blocking feature explicitly. However, unless you want to
  keep using the blocking implementation, no changes are required if you use Generator::generate.
• Snowflakes are now serialized as a u64 instead of a structure.

Happy generating!