Add 'Builder' class for master config

[stefan]

Right now the master config file expects builders to be declared as dictionaries. I'd like to suggest to replace those by actual class instances (say, 'Builder'), to make the config file more expressive, as well as more robust (the Builder constructor can check all required keywords are present).

In my master.cfg, I do:

  
  class Builder(dict):

    def __init__(self, **kwds):

      self.update(kwds)


  c['builders'] = [Builder(name='sdist', slave='cugel', builddir='sdist',
                           factory=BuildFactory([a, b, c]))]

which works nicely.

[warner]

Yeah, I'd like to move to using instances instead of dicts for builder configuration.. see #10 .

[marcusl]

This is actually a duplicate of #10