Nested Tabs: add baseParent to parent tabs

[fa137]

checks if the current state contains the base parent and sets it to active by default
for example if you have a parent state called 'test.parent', you will set up your nested tabs as following:

tabs = [
  {
    heading: 'test'
    route: 'test.parent.child1' 
    baseParent: 'test.parent'
  }
  {
    heading: 'test'
    route: 'test.cars.bmw' 
    baseParent: 'test.cars'
  }
]

childTabs = [
  {
    heading: 'first child'
    route: 'test.parent.child1' 
  }
  {
    heading: 'second child'
    route: 'test.parent.child2' 
  }
]

carTabs = [
  {
    heading: 'bmw'
    route: 'test.cars.bmw' 
  }
  {
    heading: 'mercedes benz'
    route: 'test.cars.mercedes' 
  }
]

This fixes the issue #55

[rpocklin]

Thanks for the PR. Unless i've overlooked something, $state.includes should give this functionality, and from what I can tell it works in the examples folder by highlighting Settings or Accounts as the parent route. Can you please look at that and check this functionality isn't already present?