Add internal behavior of Git

* Also updated References.
Fix glossary words styles
Add objects explanation

* Tree objects.
* Commit objects.
Merge pull request #8 from matiasbeckerle/git-concepts

Git concepts
Merge remote-tracking branch 'origin/gh-pages' into git-operations

Conflicts:
	2-concepts/index.md
Merge pull request #6 from andresmoschini/git-operations

Git operations
Update git-clients.md
Git Clients related changes in 3-working-with-git index
Add placeholders of some Git configuration files
Add reference to sources in gitignore-file