Add drakeFunction package and basic classes.

Defines the following functions
  * Affine transformations
  * Constant elementwise powers
and the following ways of combining them
  * Composition
  * Concatenation
Add drakeFunction.Sum as a drakeFunction.Container
Reimplement Sum as a regular drakeFunction.

This leaves us with only two drakeFunction.Container classes:
  * Composed
  * Concatenated
which strikes me as cleaner.
Fix Container.input_map for same_input systems
drakeFunction.Root -> drakeFunction.DrakeFunction
Add drakeFunction.frames package

Currently contains only drakeFunction.frames.R, a method which returns a
singleton frame representing R^n for the specified n.
Add drakeFunction.kinematic package

Currently contains one class (RelativePosition) and its test
Add documentation to some DrakeFunction classes
Move Container funcitonality into Concatenated.

This is the only Container we were going to need. The Container class has been
removed.
Fix missing semicolon in drakeFunction.Sum
Add drake root to path in addpath_drake

Maybe this is a bad idea? I'm doing it so that the drakeFunction package can
live in the root directory.
Fix typo in TaylorVar/power.

Previously TaylorVar/power only allowed for scalar powers of TaylorVars. Now we
should be able to do elementwise constant powers.
Add drakeFunction.kinematic.RelativeQuaternion
Add DrakeFunctionConstraint.

This class will eventually replace Constraint, but for now it allows us to
merge in new DrakeFunction features without disrupting our Constraint
infrastructure.
Add drakeFunction.euclidean package.

Currently contains a NormSquared function and test
Add methods to DrakeFunction

Added methods:
  * plus
  * uminus
  * minus
  * vertcat
  * compose
Fix Concatenated/eval in same_input case
Replace eq with isequal_modulo_transforms in Concatenated
Add SignedDistanceToHyperplane and test
Fix typos in drakeFunction.euclidean

I still had a few 'expression's in there