Version Control With Subversion Source

MOBI-formatted files (complete with a generic book cover!).

* en/book
(svn:ignore): Add 'svn-book.mobi' to the ignores list.

* en/book/images/ebook-cover.jpg
New.

* tools/Makefile.base-rules
(epub): Move UNIFIED_XML_TARGET dependency ...
(EPUB_TARGET): ... into here. Also, fix some whitespace goofs.
(mobi, MOBI_TARGET): New rules.
(clean): Also remove MOBI_TARGET.

* tools/Makefile.base-vars
(MOBI_TARGET): New.