Add an option to enable/disable case insensitive completion.

Options:
- Global option: g:jedi#case_insensitive_completion
- Buffer-local option: b:jedi_case_insensitive_completion

Values:
- 0 to disable case insensitive completion.
- 1 to enable case insensitive completion (default).

autoload/jedi.vim

@@ -39,6 +39,7 @@ let s:default_settings = {
     \ 'added_sys_path': '[]',
     \ 'project_path': "'auto'",
     \ 'smart_auto_mappings': 0,
+    \ 'case_insensitive_completion': 1,
     \ 'use_tag_stack': 1
 \ }