Adds support for site brand image

Adds an <img> tag before the SITENAME in the navbar and the variables SITE_BRAND and SITE_BRAND_SIZE.
Minor fixes to the site brand image

Changes SITE_BRAND to SITEBRAND and SITE_BRAND_SIZE to SITEBRAND_SIZE.
Also adds the missing slash between SITEURL and SITEBRAND and adds the
HIDE_SITENAME flag.
Merge branch 'patch-4' of https://github.com/magnunleno/pelican-bootstrap3 into magnunleno-patch-4

* 'patch-4' of https://github.com/magnunleno/pelican-bootstrap3:
  Minor fixes to the site brand image
  Adds support for site brand image

Conflicts:
	templates/base.html