Add an installer for the msysgit environment

This installer:
- detects the Git installation path
- checks prereqs (git, getopt.exe)
- delete old gitflow install
- install gitflow files

"Access denied" errors are detected and the user is given advice in that case (
use full administrator rights).
Update README for msysgit to use the new installer
Fixed require_tag_absent to not break when the list of tags contains an ambiguous entry, e.g. looking for "1.0.1" in "0.1.0 0.1.1 0.1.2 1.0".
- Removed quoting in has $SOME_BRANCH $(get_remote_branches), as the check was always false
- Added fetching develop branch from origin when fetch flag is on in feature finish
release: start subcommand takes an optional base as well

Signed-off-by: Stefan Naewe <stefan.naewe@googlemail.com>
Add Konstantin Tjuterev to the list of contributors.
Add Jannis Leidel to the list of contributors.
Fix always-empty value for $DOT_GIT_DIR variable.

This fixes #53.
Set GITFLOW_DIR correctly if git-flow is a symbolic link

This allows a user to create a symbolic link to git-flow in their
personal bin directory (e.g. ~/bin) without having to add the source
tree to their $PATH or do a system install.
Add debian/ubuntu packaging.
Fixing some minor mistakes in debian/control metadata - followup to ca475abb09fc761e06720b15d05f33710a17409c
Moved the debian stuff to the contrib directory.
Add Kiall Mac Innes to the list of contributors.

Thanks Kiall for the Debian package files!
Add Jon Bernard to the list of contributors.
Revert "Set GITFLOW_DIR correctly if git-flow is a symbolic link"

This reverts commit fff16edd881fee12547114d37e64d86582a9e392.
Add Olivier Mengué to the list of contributors.

Thanks for the MSysGit installer!
Make room for new minor release (and update the changelog).
Implement git flow feature finish without a branch name to finish the
current branch, if any.

This fixes #127.
Update usage docs and changelog.
Update the changelog.