0.4

This version mostly brings the new Period class and improves performances overall.

• Adds the Period class, which basically is a datetime-aware interval.
• Makes the format() method support a locale keyword argument.
• Changes custom directives. %P becomes %_z and %t becomes %_t.
  Basically, all new custom directives will be in the form %_{directive}
• Fixes singular for negative values of intervals.