Add execution of additional post-receive hooks

As this plugin occupies the post-receive hook of a managed repository,
no other post-receive hook can be used. To get around this limitation,
this commit changes the installed hook in such a way that it looks for
additional executable files in a sub-directory of the repository's hook
folder. This sub-directory has to be called "post-receive.d". The
plugin's hook will execute all the files in there marked as executable
and will provide the processes with the same STDIN stream it got when
called in the first place.
Add execution of additional global post-receive hooks

Up to now the plugin executed additional hooks when they lived in the
'postreceive.d' folder in the hooks folter of a repository. This allwed
only for either local or global hooks. To support both use cases at a
time, this commit will do two things:

1. The plugin will check whether there is a global hook directory
   'post-receive.d' present. If not, the plugin will create it in the
   common hooks directory of gitolite. Gitolite will link it from every
   repository automatically.

2. It will execute executables in the folders 'post-receive.d' and
   'post-receive.local.d', if present. The first one is meant to be
   linked to a common folder (see 1.) in gitolite's common hooks
   folder. The second folder can be used to have additional local
   per-repository hooks.
Merge branch 'feature/extra-post-receive-hooks' into feature/extra-post-receive-hooks-0.6.1

Conflicts:
	contrib/hooks/post-receive.redmine_gitolite.rb
	lib/git_adapter_hooks.rb
Merge pull request #20 from n-rodriguez/feature/extra-post-receive-hooks-0.6.1

Feature/extra post receive hooks 0.6.1