Add tests for FileTypeResolver
Add interface FileTypeResolverInterface
Implement FileTypeResolver
Rename FileTypeResolver to MimeTypeResolver
Delete FileTypeResolverInterface
Rename test class and unmark incomplete
Modify test for results with mime-type-encoding
Add resolving by extension.

e.g. hoge.jpg => image/jpeg
Merge pull request #107 from toiee/file-type-resolver-interface

File mime-type resolver