Added multi select checkbox functionality

[bindalkumar]

@alexcurtis please review. I have created this new PR to allow user to use multi select checkbox functionality.

In order to use this functionality user will have to pass following props:

<Treebeard data={stateData}
  ....
  enableCheckbox={true}
  handleCheckbox={this.handleCheckbox}
  checkboxField="name"
/>

enableCheckbox

If it is true the checkbox will be displayed. Default to false.

checkboxField

This is field which will set the checkbox value. This can be picked from data object. Defaults to name

handleCheckbox

Callback function when a checbox is checked / unchecked. Passes 2 attributes: the data node and it's checked boolean value.

handleCheckbox(node, isChecked) {
 node.checked = isChecked;
 this.setState({cursor: node});
}

[coveralls]

Coverage decreased (-0.4%) to 98.127% when pulling de11f04 on bindalkumar:multiselect_checkbox into 9069701 on alexcurtis:master.

[coveralls]

Coverage decreased (-0.4%) to 98.127% when pulling de11f04 on bindalkumar:multiselect_checkbox into 9069701 on alexcurtis:master.

[coveralls]

Coverage decreased (-0.1%) to 98.44% when pulling 0d886d5 on bindalkumar:multiselect_checkbox into 9069701 on alexcurtis:master.

[coveralls]

Coverage increased (+0.03%) to 98.58% when pulling e42ddd9 on bindalkumar:multiselect_checkbox into 9069701 on alexcurtis:master.

[bindalkumar]

@alexcurtis getting following errors while integration: