v0.2.4

Changes:

• Fix: explicitly require serde_derive when using serde feature;
• Fix: add owned string deserialization support (some serializers don't support temporary statics);
• New: Utc2k::month_abbreviation;
• New: Utc2k::yesterday;
• New: Utc2k::tomorrow;
• New: Utc2k::with_time;