0.3.0

Backward incompatible changes:

- `upload_asset()` now does just the basic upload and other functions
  must be used to fetch the relevant parameters. See the help docs.
- `release()` now outputs the tag name.

Misc additions and fixes:

- Better mimetype pattern matching.
- Guess mimetype for txt and svgz files.
- Quote values to prevent casting into numbers and booleans when
  creating JSON.
- Add support for easily creating nested JSON structures.
- Fix URL parsing on OS X.

Function additions:

- List all orgs.
- List pull requests.
- List team members.
- Add comment.
- Close issue.
- Create pull request.
- Edit pull request.