Use 'git diff' instead of diff

  - remove unneeded functions
  - add clear and toggle function
Support clear function
update document
Merge branch 'use_gitdiff'
Add custom variable for gutter window size.
Change git-gutter:enabled as buffer local variable.

Because we should have this information each buffer.
update Copyright year
support multiple length change sign
rename to good name
update document and add screenshot
refactoring view-diff-infos function
Support buffer-local and refactoring
Fixed issue #1

  - exec 'git diff' at repository root directory
update README.md
add new screenshot