bug fixes & documentation

 * Added a requirements section to the Readme
 * export: print error if key not set
 * npm scripts: fixed postmerge and update scripts
 * join: name and email now required
 * join: no default if name and email are not set globally or locally
 * Updated default key server

Tutorial, screencast & screenshot

New workflow

This commit provides a mean to:

  1. Add yourself as a contributor, and share your GPG pubkey
  2. Sync the pubkeys for a given project

Until now, how to share and sync pubkeys was entirely left to users;
while users may still decide to do everything manually, this new
process should simplify the adoption of git signing.

First commit

Base binary, code, and documentation.