Add eunit test for private_dir/0 (and cleanup some whitespace).

[seancribbs]

This is a follow-up to AZ1072, adding an eunit test for the private_dir/0 function.

[jaredmorrow]

Doesn't compile on builders: http://buildbot-master.den.basho:8010/builders/test-riak_core/builds/55/steps/shell_3/logs/stdio

To reproduce, clone into a name that isn't the : git clone git@github.com:basho/riak_core builder

[seancribbs]

This comes from an -include_lib that is broken, see https://gist.github.com/1753157. Needs to be fixed, but not related to this PR.

[seancribbs]

@jaredmorrow Fixed all include_lib statements of riak_core header files so they use include instead. Builds properly now on a checkout to a differently-named directory.

[jaredmorrow]

+1 compiles cleanly now on buildbot http://buildbot-master.den.basho:8010/builders/test-riak_core/builds/56