Make the list of modules loaded by Prelude configurable.

Modules loaded by Prelude can now easily be adjusted via
the file `prelude-modules.el` in Prelude's installation folder.
To disable a module simply comment it out; conversely uncomment
a module to enable it.
Auto-install slim-mode.
Map Command to Meta as well.
Remove dubious workaround for package.el timeout problem.
Fix #255.

`prelude-duplicate-current-line-or-region` was broken after the introduction of
dash.el.
Fix #261 - add missing dot in directory name
Display truly "random" tip on startup
Open ansi-term buffer in another window
Introduce prelude-visit-ielm command and bind it to C-c C-z (a-la-slime)
Merge branch 'master' of github.com:bbatsov/prelude
Improve the behaviour of the visit-term command
Merge branch 'master' of github.com:bbatsov/prelude

Conflicts:
	core/prelude-core.el
Improve the behaviour of the visit-ielm command
update the command for installing aspell
Merge pull request #262 from korthaerd/master

update the command for installing aspell
Add undo-tree to the default Prelude packages
Merge branch 'master' of github.com:bbatsov/prelude

# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
#
# Lines starting with '#' will be ignored, and an empty message aborts
# the commit.
Fix #264 - disable autoindent on yank for python-mode
Enhance git support

Added a couple of git related modes to the default distribution:

* git-commit-mode
* gitconfig-mode
* gitignore-mode
Add a sensible .projectile file to the project