test if preceding element can be lookBack()-ed
FIXED: preceding element can't be retrieved with lookBack()
Merge pull request #74 from zolaemil/fix-lookback

Fixed: lookBack() call now retrieves an element immediately preceding it.

Prior to this fix, the immediately-preceding element could not be returned.
Tests: Port lookBack test case for immediately-preceding element to C++
Fixed: lib naming issue

Never versions of lib boost have gotten rid of the "mt" moniker for multithreading.  setup.py now looks for both,