Support searchtools override regardless of Sphinx version

This takes the underlying searchtools.js_t out of the Sphinx distribution
path, patches it, and then fills the context and parses it as a templated
javascript file. This allows us to remove the script initialization on all
versions of this file.

The initialization block on searchtools.js_t has not changed in 10 years, so
this method should be safe. If the block changes in the future, tests will grab
this as we add new versions of Sphinx to our testing.

This reorganizes some repetitive code and cleans up a few other pieces as well:

File copying is linked to the standard copy_static_files that is run from
the builder. Running from an event on build finished was producing files without
the template context. This is because build-finished is always triggered, but
the static files are not always copied (and therefore did not have the same
template context?)

Fixes #25
Refs readthedocs/readthedocs.org#2708

.travis.yml

@@ -1,15 +1,18 @@
 language: python
 python:
- - 2.7
+  - 2.7
+  - 3.4
+  - 3.5
+  - 3.6
 sudo: false
-env:
- - TOX_ENV=lint
- - TOX_ENV=py27
- - TOX_ENV=py34
+matrix:
+  include:
+    - python: 2.7
+      script: tox -e lint
 install:
- - pip install tox
+  - pip install tox-travis
 script:
- - tox -e $TOX_ENV
+  - tox
 notifications:
   slack:
     rooms: