Various fixes, and a feature

Things this PR fixes:

* Rotations now work as expected, and compose (fixes #95)
* Fixes in path parsing
* Fixes to transformations with paths

And adds the `Mirror` feature:
* `Mirror` is an <angle, point> pair denoting the axis to use for mirroring a picture.

API
```julia
compose(context(mirror=Mirror(0, 0.5, 0.5), ...)
```

TODO:

* Make sure nothing broke in Gadfly