Statements-manager

- Update: README (53)
- FIX: The way of getting Google Docs API credentials
- ADD: The way of writing statement files
- ADD: Documentation for the uniqueness of `id` parameter

- Added feature: problem-**set** PDF output (43)
- Problem-set PDF contains all problems statements in working directory.
- Repagination are applied in the end of each problem.
- It would be available only when you execute `ss-manager run WORKING_DIR -o pdf`

- Add `--output` (`-o`) option
- `html` (default): output HTML file (existing function)
- `md`: output Markdown file 37
- `pdf`: output PDF file 38

- Enable to select language mode (36)
- Enable to attach additional files to the statement file (30)
- Abandoned specifying HTML / CSS template (31)
- always adopts default theme
- New feature: registering credentials (25)
- we do not have to specify the path to credentials / tokens manually in setting file
- Modified README
- added a screencast. this shows the behavior of this application concisely!

- edit README (28)
- modify a name of problem config (28)
- `ss-config.toml` -> `problem.toml`

- modify README so that users can install this from `pip`
- delete `requirements.txt`