Added alt=image in dtbook-to-epub3.xsl

Conflicts:
	src/main/resources/xml/schema/nordic2015-1.sch
preserve empty `img/@alt`, otherwise set `img/@alt` to 'image'
added alt="image" to relevant xspec tests for dtbook-to-epub3
Merge branch 'issue-104'

fixes #104
closes #106
allow thead and tfoot in HTML

see discussion on #105
Merge branch 'allow-html-thead-tfoot'
Merge branch 'master' of https://github.com/TamJ/nordic-epub3-dtbook-migrator

Conflicts:
	src/main/resources/xml/xslt/dtbook-to-epub3.xsl
Added latest changes to dtbook-to-epub3.xsl