Added plugin to upload generated gallery to Amazon S3
Improvements for upload to s3 plugin

- All files in the sigal build directory are now uploaded instead of manually building file list. This makes the plugin more robust for future changes.
- Added progressbar to check upload progress
- Added option to set a S3 permission policy (for example public-read or private) on all uploaded files
- Added option to skip files that were already uploaded before
- Added plugin to documentation
- Added plugin settings to default configuration template
Merge branch 'pr/114' (ref #114).