bugfix: updated bitbucket oauth authentication to work correctly with consumer key and secret

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
improvement: limit file permissions for config file (default: ~/.git-issues.conf to 0600)

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
add security information and dependencies to readme

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
improvement: save configuration options under the groupname of each provider

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>