Check the class of the argument of constructor
Indent
Add epub_book getter
Support initialization that use local file path
Add the definition of 'EPUB::Book' class name constant

Bacause:
NameError: uninitialized constant EPUB::Book
Use 'case' instead of 'if-else'
Support remote epub file
test: Add the test of initializer using EPUB::Book object
Use mock test for the test of remote EPUB file
Use 'require' for class name definition