Feature: New Search API

TODO:

- [x] Remove old search API (#177)

- Implement New:

  - [x] Repository Search API (#178)
  - [x] Code Search API (#182)
  - [x] Issues Search API (#184)
  - [x] Users Search API (#185)

- [x] Document `github3.search_*` methods

- [x] Document `SearchIterator`

- [x] Document `*SearchResult` classes