Added selectors and environment to jinja context

One can now conceivably do selection with jinja:

eg:

Selector:
```yaml
source:
  fn: pyside-qt4.8+1.2.1.tar.bz2 [unix]
  md5: 34b05faa7cc44d3c24d5ccadd894bd3c      [unix]
  url: http://download.qt-project.org/official_releases/pyside/pyside-qt4.8+1.2.1.tar.bz2
  fn: PySide-1.1.2qt474.win32-py2.7.exe      [win32 and py27]
```

Jinja Selector:
```jinja
source:
  {% if unix %}
  fn: pyside-qt4.8+1.2.1.tar.bz2
  md5: 34b05faa7cc44d3c24d5ccadd894bd3c
  {% elif win32 and py27 %}
  fn: PySide-1.1.2qt474.win32-py2.7.exe
  {% endif %}
  url: http://download.qt-project.org/official_releases/pyside/pyside-qt4.8+1.2.1.tar.bz2
```