Fix #66: add += operator to VectorIterator class

It seems that the templated std::sort with clang on OS X 10.9
requires iterators to have a += operator defined. Adding the
operator fixes the broken build.
Fix #66: add -= operator and fix assert

Fix the assert statement per recommendation by @sherm1 and
add -= operator.
Merge pull request #69 from scpeters/issue_66

Fix #66: add += and -= operators to VectorIterator class (needed by OSX 10.9's std library).