Fix Export to HTML with custom CSS style

The filename is now retrieved correctly from the CSS stylesheet URL.
Resolves #33
Updated czech translation for v0.6.0

Credits to Pavel Fric

Resolves #28
Increase version to v0.6.1
Add missing license header to new files
License of fontawesomeicon plugin

Add information about 3-clause BSD license of fontawesomeicon plugin.
Resolves #32
Update README

Add info about new version to README
Merge branch 'hotfix/v0.6.1'