Wrap prerequisite package installation commands

The new GitHub layout appears to limit the line length of code blocks
to 80 characters.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Add 'libevent' to prerequisite packages

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Merge pull request #56 from jvirtanen/fixes/documentation

Add 'libevent' to prerequisite packages