Allow tilde expansion for htpasswd file

Call os.path.expanduser on the location given by the config parameter.
This will allow to use settings like
  htpasswd_filename = ~/.config/radicale/users
Fix #143

Since all iCalendar objects have VCALENDAR inside them, that tag is
absolutely meaningless (in this context).
Merge pull request #169 from untitaker/issue143

Fix #143
Merge pull request #160 from red-hood/master

Allow tilde expansion for htpasswd file
Use warning instead of warn for logging (fix #170)