snapshot: first pass at newest-wins functionality.
snapshot: add cli flag for taking newest during merge
snapshot: keep a tab of seen packages and only include the latest copy during merge
snapshot: Move 'newest' logic out of main compilation loop and evaluate the reflist afterward.
snapshot: newest -> latest
snapshot: merge adjustments for -latest flag
snapshot: alter result generation in Merge() to accommodate snapshot diff
Initial pass at testing merged snapshotting with -latest flag
snapshot: simplify merging latest packages
Updated man page for snapshot merging with -latest flag
cmd/snapshot_merge: reword -latest flag to match man page
snapshot: break out FilterLatestPackages to its own function
snapshot: explicity call FilterLatestRefs() where needed rather than calling from Merge()
snapshot: separate test for FilterLatestRefs
snapshot: FilterLatestRefs returns nothing as it deals with a pointer.
Added CLI test for snapshot merge with latest flag
Merge branch 'f-newestpkg' of https://github.com/ryanuber/aptly into ryanuber-lastest-merge
Move command description from man file to command help. #42

Man file is auto-generated from inline command help.

N.B. I should document this in Contributing.
Simplify implementation (and improve performance) by using the fact that RefList is always sorted. #42
PackageList doesn't allow duplicates and PackageRefList does sorting on keys. #42