Add gokart build method

[vaaaaanquish]

example

import luigi
import gokart

gokart.add_config('./conf.ini')

class TaskA(gokart.TaskOnKart):
    x = luigi.Parameter()

    def run(self):
        self.dump(self.x)

class TaskB(gokart.TaskOnKart):
    t = gokart.TaskInstanceParameter()
    x = luigi.Parameter()

    def run(self):
        t = self.load('t')
        self.dump(','.join([t, self.x]))

task = TaskB(t=TaskA(x='hello'), x='world')
output = gokart.build(task)

print(output)    # hello, world

[vaaaaanquish]

@hirosassa thx.

@Hi-king @mski-iksm plz review!

[vaaaaanquish]

@Hi-king

My main concerns are about overlappings between gokart.run and gokart.build

I don't think there's usually a use case for that. So, I'll write some documentation.

[Hi-king]

I don't think there's usually a use case for that. So, I'll write some documentation.

My point is not for usage, but for developing from the point of view of the reusability. Anyway current implementation is more reusable by importing functions from gokart.run :)

[Hi-king]

@vaaaaanquish cool refactoring. Merged. Thx :)