Updated README, added os & db version tests, updated test library for 10.13

tests/test_highsierra.py

@@ -38,6 +38,20 @@ def test_init():
     assert isinstance(photosdb, osxphotos.PhotosDB)
 
 
+def test_db_version():
+    import osxphotos
+
+    photosdb = osxphotos.PhotosDB(dbfile=PHOTOS_DB)
+    assert photosdb.get_db_version() in osxphotos._TESTED_DB_VERSIONS
+
+
+def test_os_version():
+    import osxphotos
+
+    (_, major, _) = osxphotos._get_os_version()
+    assert major in osxphotos._TESTED_OS_VERSIONS
+
+
 def test_persons():
     import osxphotos
     import collections