Fix collapse icon and add expand logic

[TheChrisTech]

This PR fixes two things:

1. The Expand/Collapse Icon was always present, even if there were no child nodes. Now, this expand/collapse icon only shows when there are child nodes. Expand/Collapse Icons Should Not Render When No Children Nodes Are Available #20
2. Additional logic was added in that if the data passed to the tree includes an expanded field value of true, then that node will automatically be expanded upon loading of the tree. States #22 Add the ability to specify whether a node should be initially expanded  #23