Changed logo_name option to take an arbitrary string.

[willvousden]

This addresses #55 by removing the hard-coding of the value of project when using the logo_name theme option. Instead, logo_name is interpreted as a string, which is displayed directly.

Other options can be referred directly using the appropriate variable in conf.py; e.g.:

html_theme_options = {
    'logo_name': html_title_short,
}

[Benjamin-Lee]

Can we make this a thing? I was surprised that logo_name could not accept a string.