[nemo-qml-plugins] Add contact name groups and NameGroupModel

These are used to categorize each contact into groups depending on
the first letter of each contact name.
Merge pull request #144 from blammit/first-letter-search

[nemo-qml-plugins] Add contact name groups and NameGroupModel
[nemo-qml-plugins] Disable sparql usage in contacts models
[nemo-qml-plugins] Remove caching of displayLabel in customLabel field

Ccahing to be restored later...
[nemo-qml-plugins] Ensure phone number lookup uses normalized form