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 
can't use namespaces in the header file. This will expose every user who includes this file to this namespace unexpectedly.