Enable tree data as an object using the hierarchy of its properties

[stockiNail]

Fix #81

This PR is implementing the feature to set the tree of the data as an object. This enables to use native hierarchy.
The normalization is done using the keys of the object as possible groups, in order to apply the group by on the data.

{
  A: {
    C: {
      value: 1
    }
  },
  B: {
    D: {
      value: 1
    }
  }
}

is transformed to an array, manageable by the treemap controller,

[
  {0: 'A', _leaf: 'C', value: 1},
  {0: 'B', _leaf: 'D', value: 1},
]

where 0 and _leaf are keys you can use in the groups options, to group by,

datasets: [{
  key: 'value',
  groups: [0, '_leaf'],
  ...
}]

It also adds an additional options, treeLeafKey, where you can set the name of the property where storing the leaf node name. By default is _leaf.

• waiting for approval (and merging) of PR Use isObject from CHART.JS helpers instead of an internal one #99

[kurkle]

Looks good, could you add some samples too?

[stockiNail]

Looks good, could you add some samples too?

Added