FIX Unset internal metadata on TreeDropdownField setValue()

Otherwise the hidden field value and metadata will get out of sync.
Most commonly used together with setTitle('') as well.
FIX Don't silently unset ParentID in "add page" dialog

Happened when changing the page type, because the "selectedNode"
property isn't set in all cases
API Make Object::config use late static binding

Can now be used in instance scope, like:
  $this->config()->db
and in static scope, like:
  Page::config()->db
Solved json obj to array issue
Simplified travis builds (external setup scripts)
More straightfoward travis include/exclude
Minor: Update "Editing Online"

Adding emphasis to the "Edit this page" feature, and ensuring it comes before the more difficult option of locating the .md file manually.
Merge pull request #1348 from danrye/patch-3

Minor: Update "Editing Online"
Updated bugtracker links
Contribution guidelines, new bugtracker links
FIX Saving of custom meta tags (fixes #8288)

This should allow custom meta tags to be saved in the admin panel.

Also, if you use html5 video or audio the source sub tag
would have been stripped two these are all the tags that may be empty
and are valid.
Ensure composer files aren't accessible using IIS