Improved installer

Improved git-flow installer:
* Kept it backward compatible
* Added environment variable REPO_BRANCH_NAME to gitflow-installer.sh to allow installing specific branch
* Example hooks are installed into /usr/local/share/gitflow/barebone-hooks
* gitflow-installer.sh is using temporary directory for git repo instead of the current directory
* Cleanup at the end of installation

**Please do not forget to update the wiki**
* Environment variables that can be used during the installation are not mentioned anywhere.
* https://github.com/petervanderdoes/gitflow/wiki/Reference:-Hooks-and-Filters