Add function for accessing new/existing custom fields of a card by name

[JuergenBS]

As enhancement for the new custom field functionality I added a new function for getting a proper custom field instance for a specific card - only by specifying the custom field name.

Examples:
cf = card_obj.get_custom_field_by_name('MyText')
cf.value = u'This is my test text'
cf = card_obj.get_custom_field_by_name('MyBool')
cf.value = True

[sarumont]

Looks good, but there should be a null check and fetch before processing the local data, as self.customFields may yet be None.

[JuergenBS]

I added lazy loading for card.customFields. The data is now actively fetched on first access or the json_obj provided by from_json() or fetch() is used. I hope, this change doesn't break anything.

[sarumont]

Looks good, @JuergenBS - thanks!

[james-d-f]

Could we add something similar to get the custom field definitions for a board by name, or would that not make sense?

[sarumont]

@jamesf-brainlabs Yeah, I think that makes sense. Having an accessor would make unravelling that a bit more efficient.