Update and reorganize README.

Add an install-dev-dependecies script instead of instructions, describes
branching structure, and emphasizes Getting Started instructions.
Fix pre-push testing.
Add back libxslt1-dev requirement.
Add more dependencies.
Fix Markdown code formatting and Transifex reference.
Merge branch 'better-docs' of github.com:jsha/https-everywhere into better-docs
Merge pull request #502 from jsha/better-docs

Update and reorganize README.