Add initial PGF/TikZ drawing capability

- Line, Polygon, and LineWidth primitives seems to work
- Stroke and fill colors also work, but still needs
  heavy work
Merge branch 'master' into pgf_backend
Add more primatives

- Rectangle, circle, ellipse
- Line joins, caps
- Opacity, visibility
Add SVG/JS stubs
Add `idx` parameter `draw` functions in PGF backend
Add initial text support

- Still have issues with font size
Fix font size and text visibility in PGF
Add support for ClipPrimitive in PGF
Add text formatting to PGF plots using pango
Move `pango_to_pgf` to `pgf_backend.jl`
Merge branch 'pgf_backend' of https://github.com/darwindarak/Compose.jl into darwindarak-pgf_backend

Conflicts:
	src/Compose.jl
Trailing whitespace.