Move most of configuration from ~/.gitflow_export to natural config

This PR is based on PR #85, let me know if you need it to be rebased on develop instead.

"Pros" for moving the settings to a natural git config file:
 - Natural separation between local/global/system configurations
 - Less files to manage
 - Less OS dependencies
 - Existing management mechanisms (git config add/remove/list etc.)

Changes:
 - Moved configuration for "showcommands", "init", "feature", "hotfix", "release" and "support" from ~/.gitlfow_export environment variabels to natural git config (.git/gitflow_config). An example of how the configuration can be accessed/altered: `git config --file=.git/gitflow_config feature.start.fetch true`
 - When old environment variables are detected, git flow will exit with a message asking to update the config.