add native Windows compiler support

For now, this is just a proof of concept; would make sense to add a lot
of things, such as quick access to invoking rebar itself (with approproate 
code reloading), as well as an eunit-aware version so that tests could be 
run interactively.

By default, executables in paths such as /bin and /usr/local/bin have
the mode saying they can be executed by all.

The current version of escriptize only sets u+x, which creates problems
when copied directly in repositories and requiring other programs to
interact with them.

This change makes rebar follow the standard of linuxes and unixes by
setting the permission flag to a+x, allowing users, the group and others
to execute it.

'Erlang' projects that do not contain any erlang files (Joxa, LFE,
Elixir, etc) break the eunit task. It attempts to copy an empty list
of source files to the .eunit directory. This change makes copying an
empty list a simple no-op.

Make sure files from erl_opts->src_dirs are copied to .eunit for cover
inspection.

Without this patch cover failed to find source files which were in
src_dirs, and threw the following exception:

ERROR: eunit failed while processing
/path/to/project:
{'EXIT',{{badmatch,{error,no_source_code_found}},
         [{rebar_eunit,'-cover_analyze/3-fun-0-',1,
                       [{file,"src/rebar_eunit.erl"},{line,280}]},
          {lists,foreach,2,[{file,"lists.erl"},{line,1262}]},
          {rebar_eunit,cover_analyze,3,
                       [{file,"src/rebar_eunit.erl"},{line,279}]},
          {rebar_eunit,eunit,2,[{file,"src/rebar_eunit.erl"},{line,140}]},
          {rebar_core,run_modules,4,[{file,"src/rebar_core.erl"},{line,425}]},
          {rebar_core,execute,5,[{file,"src/rebar_core.erl"},{line,359}]},
          {rebar_core,process_dir0,6,[{file,"src/rebar_core.erl"},{line,223}]},
          {rebar_core,process_dir,4,
                      [{file,"src/rebar_core.erl"},{line,130}]}]}}

Now all source files are copied to .eunit.

These functions will be necessary in rebar_eunit.erl, too.