Make rel/data directory optional for apps that do not need it [JIRA: TOOLS-140]

[jaredmorrow]

Some apps won't have a rel/data directory like Riak, and node_package should work even without one. A user attempted to use node_package and ran into this error on debian.

cp: cannot stat `rel/snip/data/*': No such file or directory
make: *** [install] Error 1
dpkg-buildpackage: error: fakeroot debian/rules binary gave error exit status 2
debuild: fatal error at line 1357:
dpkg-buildpackage -rfakeroot -D -us -uc failed
make[3]: *** [default] Error 29
...
make[2]: *** [bootstrap] Error 2
...
make: *** [package] Error 2

[marksteele]

+1