Added support for sys.prefix based udunits xml path

The way we currently use udunits from Iris relies on it to looking in a default location for a set of xml files describing the units. I like this, but if you install Iris (and udunits as one of its dependencies) via conda in a virtual env like location the xml files will not be in this location. This should ideally be fixed in udunits, but they expose an API ```ut_read_xml()``` to enable user code to look in different locations. This PR adds a fallback if the default lookup fails. It does not break the user's ability to set a custom location via the UDUNITS2_XML_PATH environment var.

See https://github.com/nbren12/conda-recipes/issues/1 and https://groups.google.com/a/continuum.io/forum/#!topic/conda/8yZ3rxED2lk for more discussion.