Basic Alembic support

All in all, this patch adds an Alembic importer, an Alembic exporter,
and a new CacheFile data block which, for now, wraps around an Alembic
archive. This data block is made available through a new modifier ("Mesh
Sequence Cache") as well as a new constraint ("Transform Cache") to
somewhat properly support respectively geometric and transformation data
streaming from alembic caches.

A more in-depth documentation is to be found on the wiki, as well as a
 guide to compile alembic: https://wiki.blender.org/index.php/
User:Kevindietrich/AlembicBasicIo.

Many thanks to everyone involved in this little project, and huge shout
out to "cgstrive" for the thorough testings with Maya, 3ds Max, Houdini
and Realflow as well as @fjuhec, @jensverwiebe and @jasperge for the
custom builds and compile fixes.

Reviewers: sergey, campbellbarton, mont29

Reviewed By: sergey, campbellbarton, mont29

Differential Revision: https://developer.blender.org/D2060

release/scripts/modules/sys_info.py

@@ -158,6 +158,13 @@ def write_sysinfo(filepath):
     else:
         output.write("Blender was built without OpenVDB support\n")
 
+    alembic = bpy.app.alembic
+    output.write("Alembic: ")
+    if alembic.supported:
+        output.write("%s\n" % alembic.version_string)
+    else:
+        output.write("Blender was built without Alembic support\n")
+
     if not bpy.app.build_options.sdl:
         output.write("SDL: Blender was built without SDL support\n")