Correct message when called without a parameter
Return 2 in case of incorrect usage

Just for the sake of consistency.
Add log_error/1
Add paths and includes to fix plain app projects

So far we found code paths and includes for project layouts (where we have an
"apps" directory and rebar.config lists the application directories).

Now the plugin also works for plain app projects (where src, ebin and include
are all next to rebar.config).
Merge pull request #10 from hcs42/master

Fix adding paths and includes for plain app projects