Default to the thing (hopefully, package name) at point

* Also prohibit non-matching input.
* Unfortunately, this change also picks up any irrelevant symbol
  at point and also adds it to candidates.
  Maybe we should check if it's in the list, first.
Added advice to describe-package-1 to add a Visit Homepage button.
Make the Visit Homepage button customizable.
Reordering of code, make muv asynchronous, use lexical binding.
Merge branch 'patch-1' of git://github.com/dgutov/melpa-upstream-visit into dgutov-patch-1

Conflicts:
	melpa-upstream-visit.el
Do not pick up irrelevant symbols at point - as dgutov suggested :-)
Added thank yous.
Add two button styles: top right, overlaid over package-name.
Bump version number to 0.5.
require cus-edit instead of custom for the custom-button face.