MODE-1761 Added debug and trace logging to sequencing operations

Added debug logging for 'org.modeshape.jcr.sequencing' that outputs
the initialization of each sequencer (with valid path expressions)
and each invocation of a sequencer.

Added more trace logging for 'org.modeshape.jcr.sequencing' that
outputs very details messages describing the various activities
(and exit points) of sequencing.

Note that unconventional log names were used to make it easier
to enable and disale all logging for sequencing.
MODE-1768 Added checks to include only existing and readable files as child nodes
MODE-1767 Added logic to simplify MIME Type handling

When a session is saved, it now looks for nodes that are of type
'nt:resource' (including when the node's primary type is a subtype)
and checks whether the 'jcr:mimeType' property is set. If it is not
and there is a 'jcr:data' property with a single BINARY value, then
the session attempts to obtain the MIME type from the Binary value
and set the 'jcr:mimeType' property. (Note that name of the
'nt:resource' node's parent, if there is one, is supplied to the
'getMimeType(...)' method to hopefully get the most accurate MIME
type for the node.)

This is useful because applications should be able to access the
MIME type for 'nt:resource' nodes (e.g., the 'jcr:content' child
of 'nt:file' nodes is often of type 'nt:resource') without having
to cast to a ModeShape-specific interface.

Because the MIME types are computed asynchronously as soon as the
Binary value is created (and thus before it is set on a specific
value), the determination of the MIME type will hopefully have
completed before 'Session.save()' even tries to get the value.

Also, the 'jcr:mimeType' property is an optional property on
'nt:resource'. So whenever possible, client applications should
set this property where possible. In such situations, there is
no additional overhead to the 'Session.save()' call.
MODE-1767 Added unit tests

Added four unit tests that verify the behavior of the automatic setting of the
'jcr:mimeType' property only on nodes of type 'nt:resource'.