Fix for https://github.com/Netflix/astyanax/issues/183

Fixed issue when paging over slices that if the data is removed while
paging that it can remove the wrong element or get an
IndexOutOfBoundsException.
Fix for https://github.com/Netflix/astyanax/issues/188
Merge pull request #189 from Netflix/entitymanager

Fix for https://github.com/Netflix/astyanax/issues/188
Merge pull request #184 from wburns/Issue-183

Fix for https://github.com/Netflix/astyanax/issues/183
Merge pull request #187 from stevenzwu/master

more comprehensive handling of nullable columns