allow creating system edge and document collections.  

compat change: by default now, prefixing the name with an underscore
(ex. _) will set the isSystem flag to true instead of throwing an
exception.
added a method to get the checksum of a collection, also added a helper
method to get the isSystem flag (available as a key from getProperties()
also)
some more tests - mostly collection related, database tests updated to
include testing creating some collections...