Update deb-package requirements to include libssl-dev and libffi-dev.

libssl-dev is required for compiling the scrypt python module.
libffi-dev is required for cffi, which is a requirement for bcrypt.
Pull dependencies from install_files in setup_ubuntu.sh.
Set mysql password directly. Makes it possible to run setup_ubuntu.sh multiple times.
Modified installation instructions for securedrop development.

Installation instructions were out of order and the wording was slightly confusing. Attempted to make the instructions sequential and more understandable. Notably, inlcuded instructions about creating the GPG key before the copying and modification of the config file.
Recommend encrypted LVM
Merge pull request #271 from ternus/ternus-setup-script

Improve setup_ubuntu.sh
wrap long lines
Add warning about unattended-upgrades
Merge pull request #272 from kyleolivo/master

Conflicts:
	securedrop/HACKING.md

Manually merged into develop instead of master (submitter chose wrong target
when submitting the pull request).

* wrapped lines > 80 chars.
* removed recommendation to create dirs in `SECUREDROP_ROOT` manually, this is
now handled automatically in `config/base.py`
Merge pull request #276 from ageis/develop

Recommend encrypted LVM
Merge branch 'develop' into functional-tests

Conflicts:
	setup_ubuntu.sh
added comment for webdriver send_keys function