MultipleVariantReader and MultipleVariantIterator

Enables iteration over multiple input files, returning a vector of Variant objects per genomic location.

The vector is reused per location, so the caller must complete processing before continuing iteration, or make a copy.

Strongly inspired by the example of VariantReader/VariantIterator.  Merging these Readers or Iterators may be appropriate.

This is an alternative solution to Issue #76 
I attempted to use boost zip iterators but I was unable to get it working.  I was unable to find a simple "zip" function as described here.  I would love to see a working example of how to use it.