Add a string representation of UTC Offset class

[Brandon Nielsen]

Printing a date with a UTC offset involved results in an ugly memory address being printed:

<aniso8601.timezone.UTCOffset object at 0x7f040b319990>

Instead print out the + / - UTC offset value.

[Brandon Nielsen]

Fixed in a string of commits: 0f07492, 2f23891, 11e51d3, and 6bf7f18.