Fix a bug that was causing rake to overwrite SHELL settings

The prezto install procedure now checks to make sure the user
doesn't have an alternative version of zsh installed before changing
the current shell preference with "chsh".
This may be the case when an user is already using zsh but on a
different version from the one shipped with OS X

Fixes #264
Merge pull request #265 from duhanebel/zsh_install_check

Fix a bug that was causing rake to overwrite SHELL settings
Fix a bug that's causing the installer to overwrite backup files

When rake install/update is run for the second time, any .backup
file created by the first run it's going to be overwritten by
the new symlink even if the current file is the same as the one
we are going to write.

The rakefile now checks if the symlink we are going to create
points to the same target as the one already there and if that's
the case, doesn't overwrite any backup.
fixes #268
Merge pull request #268 from chadkouse/env_shell

"uninitialized constant SHELL" in zsh install step
Merge pull request #266 from duhanebel/install_link_fix

Fix a bug that's causing the installer to overwrite backup files
New: [VIM] added file-line plugin. Allows to open file and move to certain line like this vim Rakefile:110
Merge pull request #269 from petRUShka/master

Add vim-plugin for opening files like this `vim Rakefile:110`
Merged with upstream
Merge branch 'master' of https://github.com/jby/dotfiles

* 'master' of https://github.com/jby/dotfiles:
  Synced submodules with upstream
Synced modules with upstream