v0.4.2

Deprecations

• More strongly deprecate RustcSerialize: remove it from documentation unless the feature is enabled, issue a deprecation warning if the rustc-serialize feature is enabled (@quodlibetor #174)

Features

• Move all uses of the system clock behind a clock feature, for use in environments where we don't have access to the current time. (@jethrogb #236)
• Implement subtraction of two Dates, Times, or DateTimes, returning a Duration (@tobz1000 #237)