reltool is missing an app resource file before R14A.
Therefore, restore the old check and add TODO.

If running e.g. rebar generate using an OTP development build,
rebar will complain that reltool has the version "", which is
less than the required "0.5.2". This is because rebar_reltool
simply checks the path returned by code:which(reltool), which
doesn't yield version information if used in a development build.

This patch substitutes a more robust method (load reltool and
fetch the info from application:loaded_applications()).

As it happens, this will not be enough to make things work,
but now Reltool will explain that it cannot generate a spec
from a system that is not installed, giving a better hint
as to what needs to be done.

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.