updated Dockerfile:

* switch to latest Ubuntu LTS;
* install adblockparser, re2 and pyre2
docker: pin Python dependencies, move ADD ./app below
add .dockerignore (6MB of context data instead of 120+)
try starting Xvfb automatically
Dockerfile: start splash directly
Docker: allow to pass options and to change request filters, js profiles and proxy profiles without rebuilding the image.
more docs for Splash+Docker
mention Windows
clarify folder sharing docs
upstart: extra splash argument could be passed in SPLASH_OPTS env variable now.
Merge pull request #113 from sardok/extra-opts

upstart: extra splash argument could be passed in SPLASH_OPTS env variable.
Merge pull request #108 from scrapinghub/docker

[MRG] updated Dockerfile
cleanup: simplify parameter handling code by using keyword arguments
cleanup: use dicts more consistently
cleanups
cleanup for doRequest methods of WebpageRender subclasses
fail earlier in case of exceptions in doRequest
rename doRequest to start
allow to pass headers to WebpageRender explicitly
allow to pass http method and request body to WebpageRender