* add compile type for dynamic project compilation
* new rebar_compiler abstraction for running multiple compilers
rebar_compiler is a new behaviour that a plugin can implement to
be called on any ues of the compile provider to compile source
files and keep track of their dependencies.
* fix check that modules in .app modules list are from src_dirs
* use project_type to find module for building projects
* allow plugins to add project builders and compilers
Tristan Sloughter