compiler behaviour

[tsloughter]

Currently a mess, but want to start getting feed back as early as possible. And since all tests pass I figured I'd open now. will try to at least clean up the api tomorrow (callback specs and actual functions don't match because I'm letting it evolve as I see what is needed by each type of compiler, xrl, yrl, mib, erl).

[tsloughter]

Less of a mess now.

Note about the implementation:

• dependency graphs for source modules are kept separate for each compiler, cached to <app>/.rebar3/<compiler>/source.dag
• extra_src_dirs (like test) are still compiled to the directory the source is in and not to ebin but I think we should discuss changing this since the modules list in the .app is now built only from modules compiled from source files in src_dirs.

[tsloughter]

One thing still to be done in this PR is the config option for compilers.

I want to support setting compilers and resources in rebar.config instead of requiring use of plugins since these will just be module names and so the apps that provide them can be in the normal deps list.