Add "encoding: utf8" comment in order to suppress encoding error message

This error message was being generated in Rubies 1.9.2, 1.9.3, and 2.0.0.

    test_ical_1(TestVcard): ArgumentError: invalid byte sequence in US-ASCII
    /Users/h2s/src/vpim/lib/vpim/rfc2425.rb:92:in `block in unfold'
    /Users/h2s/src/vpim/lib/vpim/rfc2425.rb:88:in `each_line'
    /Users/h2s/src/vpim/lib/vpim/rfc2425.rb:88:in `unfold'
    /Users/h2s/src/vpim/lib/vpim/rfc2425.rb:315:in `decode'
    test/test_vcard.rb:351:in `block in test_ical_1'

Adding a "#encoding: utf8" comment to the top of all the source code files using
the magic_encoding gem fixes the error. This now leaves only the invalid
multibyte encoding error to deal with.