Make covertool again compatible with rebar

Associated change in rebar:

commit e185e86bff17fa7d093f8b24fcc45069ffde55ae
Author: Tuncer Ayaz <tuncer.ayaz@gmail.com>
Date:   Sun Apr 22 21:53:32 2012 +0200

    Remove shared state
Remove rebar

Not needed, since it should be executed by parent app anyway.
Merge pull request #5 from Motiejus/compat_rebar2

Make covertool again compatible with rebar
use global environment rebar executable

After removing the rebar executable it is a good idea
to point to a global environment rebar executable
for less confusion.
Merge pull request #6 from djui/patch-1

use global environment rebar executable
Merge remote-tracking branch 'upstream/master'
Fix unused variable
Fixed unused variable
Merge remote-tracking branch 'upstream/master'
Make appname configurable from command line
Merge pull request #8 from chrisavl/master

Make appname configurable when running from command line
appname parameter added to usage notes
fixed error with 'rebar_app_utils:load_app_file'
added covertool_app_name config option
skip empty folders
fix
Rebar plugin should import cover data for eunit

In newer versions of rebar cover is stopped after a eunit run. Made
rebar_covertool rely on importing the cover data instead of relying
on a running cover_server.
Merge pull request #9 from markusn/feature-rebar-eunit-cover-working

Rebar plugin should import cover data for eunit
Generate reports for functions (methods)

Requires access to beams compiled with debug_info
Handle beam_lib error case correctly