cleanups related to #637
Merge branch 'master' of github.com:PDAL/PDAL
Moving merge and sort filters
Merge pull request #639 from PDAL/move-merge-and-sort-filters

Moving merge and sort filters
Convert unit tests from Boost Test to Google Test
move gtest to vendor folder
moved the gtest source
linux is more picky about the missing include of algorithm
update docs