Don't clobber a prexisting LIBRARY_PATH on init

LD_LIBRARY_PATH and LIBRARY_PATH shouldn't get blown away during `parts init`,
instead prepend the parts related paths.  There is a chance that /usr/local/lib
and similar would get duplicated, but no harm in that case.
Merge pull request #35 from cschneid/dont_clobber_library_path

Don't clobber a prexisting LIBRARY_PATH on init