Added support for unicode cube attributes.

I have come across cf-netcdf files which when loaded into Iris give me a cube with attributes that are unicode (this is allowed under CF). These files load and save ok, but if I ```print cube``` I get a UnicodeEncodeError. For example:
```python
import iris.tests.stock
unicode_str = unichr(40960) + u'wxyz' + unichr(1972)
cube = iris.tests.stock.simple_1d()
cube.attributes['source'] = unicode_str
print cube
```
gives:
```UnicodeEncodeError: 'ascii' codec can't encode character u'\ua000' in position 0: ordinal not in range(128)```

This PR modifies ```cube.summary()``` so that it handles unicode attributes.