Bug: #98

1. dagrt.vm is now used in place of leap.vm, except for implicit
stuff, which remains in leap.

2. Tests that are also part of dagrt get removed.

3. Fix setup.py

Refactor RK code generation to use a single generator, rename 'TimeStepper' -> 'Method' (IMEX stil broken)

For some testing code that uses the old CodeBuilder, move the
CodeBuilder code to testing utils and rename as RawCodeBuilder.

Bug: #64

expressions.

That is, AssignSolved can in principle solve for multiple variables
and accept a system of equations.

Add a special case to NewCodeBuilder to handle the simple case of
1 variable and 1 equation.

Code lowering for the generalized AssignSolved instructions is not
yet implemented.

- Separate subdir for each example.
- Get the rk example running again.
- Move implicit Euler to examples (this is more of a toy method that is
  useful for experimentation than a "real" method).

Start to change the interface of a method generator (Method class) to
support two different functions: `generate` and `implicit_expression`:

 - `implicit_expression` returns an expression template that specifies
   to the user the format of the implicit expression to expect

 - `generate` replaces `__call__`, except that it takes as a parameter
   a list of *solver hooks*, which are callbacks provided by the user
   that, given a particular instance of an implicit expression, will
   return a new expression for the solve component

In order to facilitate the above changes, passing component id names
is moved to the constructor.

Provide a default solver hook (ScipySolverGenerator) for the Python
backend to demonstrate the use of the implicit tools that have been
developed.

Add tools to simplify the manipulation of expressions, including
parsing and matching.

Bug: #8