Fixing hint decoding issue (encoding to utf8) + new unittest

Rot13 translation is working but data from the webrequest was not encoded using utf8.

```
Traceback (most recent call last):
  File "geocaching.py", line 390, in test_loadCache
    c = self.g.loadCache("GC4FRG5")
  File "geocaching.py", line 303, in loadCache
    hint = Util.rot13(hint.text.strip())
  File "/Users/driquet/git/pycaching/pycaching/util.py", line 84, in rot13
    return string.translate(text, Util.rot13codeTable)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/string.py", line 498, in translate
    return s.translate(table + s[:0])
UnicodeDecodeError: 'ascii' codec can't decode byte 0x80 in position 128: ordinal not in range(128)
```

I fixed this bug and also add a unittest to test this case (with hint containing non-ascii chars).