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.
wrap long lines
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`