Fix eviction in MonoCache::find_or_create
configure: Prevent in-tree builds

Fixes #251.
Make rust-png depend on libpng

Fixes #1014.
Add CONTRIBUTING.md

Fixes #1026
Implement DocumentFragment.
Remove some type variables and an explicit dereference.
Implement Window::get_cx() to reduce code repetition.
Make Document::root private, in preparation for removing it as part of making Document a Node.
Fix parallel build with rust-auto-clean-trigger
Trigger Rust
Define "isnot()" for script test.
Pass AbstractNode to Node<ScriptView>.add_to_doc().
auto merge of #1054 : sanxiyn/servo/fix-stamp, r=jdm
auto merge of #1040 : kmcallister/servo/monocache, r=metajack
Basic Implementation of document.getElementById(), #740
auto merge of #1043 : kmcallister/servo/misc, r=larsbergstrom
auto merge of #1044 : Ms2ger/servo/DocumentFragment, r=jdm
auto merge of #1050 : Ms2ger/servo/get_cx, r=jdm
auto merge of #983 : saneyuki/servo/getid, r=jdm

This has not implemented these features yet:
* Update hashmap which is used for `getElementById()` when updating element attributes.
* Keep node order in tree.

However, this has the quality of that we can use this method for testcases.
auto merge of #1051 : Ms2ger/servo/private-root, r=jdm

In preparation for removing it as part of making Document a Node.