101 samples

…a la [Reactive Extensions](http://rxwiki.wikidot.com/101samples). [Rendered content](https://github.com/libgit2/libgit2.github.com/blob/101-samples/docs/guides/101-samples/index.md). Some ideas for content:

- [x] Errors
- [x] Advice for freeing
- [x] Repos
  - init (simple and with options)
  - open (simple and with options)
  - clone (simple and with options)
- [x] Diff
  - `git diff`
  - `git diff --cached`
  - Diff a commit to its parent
  - ~~Generating a diffstat~~ (too big)
- [x] Status
- [ ] History
  - Something like `git log`
- [ ] Rev-parse
  - Examples of different syntaxes
- [x] Objects
  - Lookups
  - Casting `git_object *`s to different types
- [ ] Refs
  - Dereferencing to commits or trees
  - Following chained tags