pkgdown 1.3.0

(This was meant to be pkgdown 1.2.1, but I accidentally submitted the wrong version to CRAN)

• Restore accidentally deleted build_logo() function so that logos
  are once more copied to the website.

• Fix to pkgdown.css so page header has correct amount of top margin.

• content-home.html template is no longer used when the homepage
  is an .Rmd (Reverts #834. Fixes #927, #929)

• deploy_site_github() now passes parameters to build_site()
  (@noamross, #922), and the documentation gives slightly better advice.

• Correct off-by-one error in navbar highlighting javascript; now no navbar
  is highlighted if none match the current path (#911).

• Tweaking of HTML table classes was fixed (@yonicd, #912)

• Restore accidentally removed docsearch.css file.