Fix #4: URL type should not have parameters

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_**Don’t merge this yet!**_
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The validation is correct.  However, it breaks the tests; the test file `maximal.vcf`, which is supposed to be valid, actually contains an invalid line; the `URL` type should not have parameters—but in `maximal.vcf`, it does.

I tried simply deleting the invalid parameter from `maximal.vcf`.  That makes the tests raise the following error:

```
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ERROR: test_valid (test.test_package.TestVCards)
Valid (but not necessarily sane) vCards
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Traceback (most recent call last):
  File "/Users/amrogers/Developer/Projects/GitHub/vcard/test/test_package.py", line 246, in test_valid
    '''.format(vcard_text, error)))
UnicodeEncodeError: 'ascii' codec can't encode character u'\xc5' in position 509: ordinal not in range(128)

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```

I believe that `u'\xc5'` is the character `Å`, which is, naturally, *not* ASCII.  However, I have no idea what’s causing this error to occur.  

Can you help me track it down?