Basic grid-based plotting.

Trees in various orientations (left/right/up/down/circle), with and
without node/tip labels.  Very limited support for non-scalar graphical
parameters yet, but the most basic plotting code is there.
Classify a tree by membership of a node.

Steps towards the MEDUSA classification algorithm.  Will be used
by plotting code to highlight clades, etc.
Support for combining graphical parameters
Simplify branch handling.

No longer store grobs for "t" and "s" axes, but just one "branches"
grob that does both.  In theory that means that we can specify different
types of branches later.
Allow extraction of terminal subtrees

In the end this *is* a good idea, even if it could lead to
some surprising situations.
Basic classification tests
Allow zero-regime classification.

Also adds test for failure on two regime classification.
Classify works with >1 node.
Style elements of a tree (branches, labels) based on node
Updated documentation with new arguments.