FIX Change build approach to "double-compile"

Previous approaches and why they don't work/are not nice:

* symlink to piqi_src on post-get-deps hook:

    post_hook for get-deps gets executed after THIS project is retrieved, not
    after its dependencies are (see [rebar issue]), therefore the symlinking
    fails.

* symlink to piqi_src on pre-compile hook:

    the first thing rebar does is scan and consult .app.src files, so this will
    cause the "fake" .app.src file to be used, because it will ignore the new
    .app.src file in the symlinked folder

* copy over .app file after compile

  - the file is static, so you will need to manually maintain modules, rebar
    does a better job at this using .app.src files
  - you also need to manually replace `{git, vsn}`

New approach: symlink on pre-compile, recompile twice on the very first compile
to ensure the right `.app.src` file was used.

[rebar issue]: https://github.com/rebar/rebar/issues/61
NEW Bump piqi_src tag to one with dynamic .app.src

This is possible after the previous commit (e406b43d9f), since the .app.src
file will be recompiled instead of just being copied over.
Merge pull request #2 from yfyf/double_app_src_recompile

FIX Change build approach to "double-compile"