let plugin declare dependencies in a metadata file

idea here is to move from build-time dependency management in clickstack to runtime-dependencies.
for sample, if I want to run tomcat7 clickstack with a custom java stack, I'd like to set PLUGIN.SRC.java=..., not rebuild tomcat7 clickstack.
this pull request let a plugin declare metadata, with support for dependencies/plugins to declare transitive dependencies. so tomcat 7 clickstack can depend on java one, not include it.

very basic implementation : only support first level dependencies (sorry, my first erlang development)