Add optional template for mktemp, so that it works in Mac too.

GNU mktemp does not require a template as a required argument while the
FreeBSD version that comes with the Mac, needs it:

GNU mktemp:
   mktemp [option]... [template]

FreeBSD mktemp
   mktemp [-d] [-q] [-u] template

I have just added a template in the form XXXXXXXX so that it works in
both environments.
Add prefix to generate tmp files in /tmp, except for the exercise repos
Change template from GITPractice_XXXXXXXX to GITPractice_Repo_XXXXXXXX
Change the template for the remote repos to GITPractice_REMOTE_REPO_XXXXXXXX
Using absolute path to call the scenario scripts.

Playing around with bash to get the absolute path to the run_tests
script, and from there calculate the path to the script.
The motivation of this, is to allow this script to be run from either
the test directory, or from any other place.
Add utils library, which should contain a collection of common functions.

Right now, the library only contains a way to get the current repository
(by reading repository.txt), so it can be used from the scenarios to
simplify the user's life.
Changes to not show the absolute path to scripts to the user.

When any scenario is called with an absolute path, this script (used
from the scenarios to place the information in the screen) will print an
absolute path too. I have just changed that, to show only the script
name.
Some identation changes.

Substituting \t by spaces.
Removed lib/utils.bash

Didn't use it in the end as I suppose I would, so I'm removing it.
Adding /tmp prefix, to force the creation of the repos in /tmp
Merge branch 'master' of https://github.com/Gianfrancoalongi/git_practice into HEAD
First non-functioning version of a script to guide the users through the tutorial
Add welcome and ending messages
Add welcome function
Add an initialization func, to group more things, despite of initializing the current_scenario
Some functions added
changing options for run_tutorial
A few adjustments
Merge pull request #1 from darkturo/master

Making this work for Mac too