Min/Expand for nodes with children

Only show the expand / minimize for nodes with children
Merge pull request #138 from enkodellc/patch-1

Min/Expand for nodes with children
Fixed #137
Merge branch 'master' of https://github.com/JimLiu/angular-ui-tree

Conflicts:
	demo/dist/angular-ui-tree.min.js
	dist/angular-ui-tree.min.js
roll back dem.'s
Fixed bug of read me #151
Merge from https://github.com/JimLiu/angular-ui-tre