Add more asserts for view dequeueing in IGListAdapter

[rnystrom]

Noticed this in #90, it looks like there are some asserts that should be cleaned up:

• Assert elementKind.length > 0 for supplementary views

  • IGListKit/Source/IGListAdapter.m

    Line 746 in ce4b851

    - (__kindof UICollectionReusableView *)dequeueReusableSupplementaryViewOfKind:(NSString *)elementKind

• Assert index > 0

  • IGListKit/Source/IGListAdapter.m

    Line 714 in ce4b851

    atIndex:(NSInteger)index {