compile_python_variable assumes that repr() of variable's value is a valid Python expression

The compile_python handler for Variable objects is:

    @compile_python.when(Variable)
    def compile_python_variable(compile, variable, state):
        return repr(variable.get())

This assumes that the repr() of the value inside the variable will be a valid Python expression (and more concretely, a valid expression with storm.expr as the globals).

There are a number of ways this can cause problems:
1. Values that have a repr() like "<something at 0xNNNN>" will give a syntax error, even if there is a compile_python handler for its class.

2. For e.g. UUID objects, while the repr is a valid Python expression, it requires that the name "UUID" be bound:

    >>> import uuid
    >>> from storm.expr import compile_python
    >>> from storm.variables import UUIDVariable
    >>> var = UUIDVariable(uuid.uuid4())
    >>> matcher = compile_python.get_matcher(var)
    >>> matcher(None)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<string>", line 1, in match
    NameError: global name 'UUID' is not defined

    >>> import storm.expr
    >>> storm.expr.UUID = uuid.UUID
    >>> matcher(None)
    True

As e.g. ResultSet.set() is only checking for CompileErrors, this causes the method to fail when it would otherwise be able to succeed.

Suggested changes:

1. update compile_python_variable to delegate compilation to the value:

    @compile_python.when(Variable)
    def compile_python_variable(compile, variable, state):
        return compile(variable.get(), state)

2. define some way for compile_python handlers to inject names into the scope for the matcher function (probably via the state variable).

3. add compile_python handlers for any variable values that don't have them currently.