Add Git support

This commit adds a new submodule for working with Git repositories
inside Vim :
  - vim-fugitive
Modify Vim status line

Modify the Vim default status line to display more useful information
like file type, Git information, etc...
Change <Leader> key
Add vim-pathogen helptags

Run the helptags function of vim-pathogen to update help tags
Add file explorer and window navigation

Add a new submodule for file/directory exploring :
  - NERD tree

As this plugin opens window splits, remap window navigation keys for
quicker access
Modify some settings

Modify settings to load NERD tree on startup, map some keys, display
extra characters, etc...
Add buffer explorer

Add a submodule to handle buffer manipulation. The added module is :
  - MiniBufExpl