Styling functions now use '+' to combine with tree grob
Test arguments to tree_style
Issues arising from the last round of work
The `+` method for adding to tree grobs is now extensible

Uses S3 methods, and some pokery to dispatch on the second argument
I.e., given tree + thing, dispatch on class of thing not of tree
(which will always be a tree grob)
Split the plotting file up.

Was getting a bit unweildly.  Hopefully this makes the similar bits
close together, but I'm not tied to the current split.
Fixes #47
Fix compilation on recent Rcpp
First draft of image plotting code.

Very sketchy and will change soon.
Basic tests, should be ready to refactor
Refactor out location code
Test the 'tree_label_coords' function
Factor out the time offset code
Load methods.

Because basically *nothing* works properly without it, making
Rscript basically useless.  Seriously annoying.
Simplify treatment of inter-tip spacing and circular plots
Basic tree_brace object

Does nothing so far except allow construction.  "add" coming next.
Add a single brace to a tree.
Plotting progress towards multiple braces.

Fair bit of refactoring needed still
Abstract out some of the tree lines code

tree_brace works better now, and branches is a bit more robust.