fix UnicodeEncodeError for non-ascii data

This fixes the UnicodeEncode error when loading `xero.contacts.all()` with non-ASCII data:

```
Traceback (most recent call last):
  File "test.py", line 10, in <module>
    print xero.contacts.all()
  File "/Users/shamrin/src/xero/pyxero/xero/manager.py", line 154, in wrapper
    dom = parseString(response.text)
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/dom/minidom.py", line 1931, in parseString
    return expatbuilder.parseString(string)
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/dom/expatbuilder.py", line 940, in parseString
    return builder.parseString(string)
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/dom/expatbuilder.py", line 223, in parseString
    parser.Parse(string, True)
UnicodeEncodeError: 'ascii' codec can't encode character u'\u2013' in position 13093: ordinal not in range(128)
```

(No tests for now because of #4)