Use CodeLoader.source_extension abstraction everywhere

There were still some parts of codeloader using hard-coded ".rb" instead
of the source_extension abstraction. Every occurrence has been replaced.
Add specs for File.realpath with invalid inputs
Fix File.realpath with invalid inputs. Fixes #2165
Translate table of contents into Japanese
Add myself to AUTHORS
Merge pull request #2211 from inukshuk/patch-1

Add myself to AUTHORS
Merge pull request #2199 from programble/patch-1

Use CodeLoader.source_extension abstraction everywhere
Merge pull request #2212 from utenmiki/translate

Translate table of contents into Japanese
Merge pull request #2210 from kachick/fix-file-realpath_should_raise_error_if_invalid_path

Fix File.realpath (for #2165)
Improve spec descriptions.
Removed BOM so Jekyll will process the file.
Regen'd website.
Change type for constant expressions to Constant
Create better KeyError if hash key is missing
Add support for 1.9 named hash arguments in sprintf

We use the same parsing for 1.8 and 1.9 mode, just the regular
expression is different so the new 1.9 style is never matched by the 1.8
regexp. This makes for less duplicate code, while supporting both modes
properly.

Fixes #1627
Remove tags for passing sprintf specs
Update tags for 2.0 mode
Add specs for Regexp#match and String#match
Use character offsets for String#match and Regexp#match

Fixes #1965