Support for: nested source directories, improved deps handling, improved...

... .hrl prerequisite checking, release handling

Hi Pierre,

Have been using erlang.mk for a while, but had some problems with it (main one being an inability to handle .hrl dependencies particularly well) and I came across your erl.mk repo.  Good job!  It's a great makefile that does a good deal of things nice and simply.

I've done some fairly major refactoring on the .erl compilation to support the '-M' flag to erlc, providing a generated file with the exact correct prerequisites for each .erl.  In doing that, I also tweaked the compile to build all the .erl's in one go rather than executing erlc multiple times - that gives a significant performance advantage.

Whilst doing that, I also played with the dependency handling and added in the release stuff from erlang.mk.

It's a pretty big pull request; frankly, the diff is close to useless and it's probably easiest to just read my version of the makefile.  If you like it, then feel free to pull - comments and suggestions much appreciated!

Cheers,

Steve