fixed handling of custom LEANPUB_BOOK_OUTPUT_DIR

This fixes Issue #3 (closed)

It turns out there already WAS support for specifying a custom directory but it didn't work. This fixes all the bugs I encountered.

At a high level there were two issues:

1. org-leanpub-markua-export-to-markua & org-leanpub-markua-export-to-markdown were never being told where the outdir was. This results in errors when it tries to write to manuscript/foo.markua because there's no manuscript directory.
2. A symlink was being created from images -> <outdir>/resources/images This is a problem because one compelling reason to specify a custom output directory instead of just taking the default is because you have multiple books in the same repository. When this is true the images symlink is only pointing to images folder of the last document you exported which is problematic when you've got multiple export folders.

In addition to fixing these issues, I've made a stab at documenting this functionality and the slight difference in expected results.