FEATURE: support multiple addon-sdk

This change needs tweaks on Gruntfile options syntax:

- "mozilla-addon-sdk" multi-task is used to download and use multiple addon-sdk
  archives:
  - from official releases from ftp.mozilla.org by default, using
    an sdk version as "revision" attribute (e.g. "1.14")
  - from github archives, using the new "github" boolean attribute and
    a commit sha1, a tag or a branch name  as "revision"
    attribute (e.g. "master")
- new "mozilla-cfx-xpi" multi-task is used to build multiple addons,
  using one of the downloaded addon-sdk (using its name in the
  "mozilla-addon-sdk" attribute
- add support to multiple mozilla-addon-sdk to the "mozilla-cfx" command
FEATURE: add supports for addon-sdk github forks

- optional customizable github_user attribute
updated README.md (and syntax examples in the Gruntfile.js)
prevents dots in the multitask target names
updated README.md