Add a default Client.context property

related to feature request #40

It still needs some documentation before merging.
The recommendation is to set client.context with a new dictionary, and to avoid updating it. (Because all existing Record and RecordList share the same context)
If the default context needs to change, then copy the old one and replace the client property.

Example:

```python

# Set language to German
client.context = {'lang': 'de_DE', 'preferred_color': 'blue'}
# ... do something

# Switch to Italian
client.context = dict(client.context, lang='it_IT')
# ... do something

# and AVOID:
client.context['lang'] = 'fr_FR'   # NO, because it updates the context of all previous Record and RecordList
```