Added a mechanism to fetch available per-distro architectures

The architectures are fetched from a arches.yaml file, located
in rosditro/releases/ (alongside with targets.yaml)
Will take into account in a later commit.

The format of this file is as follows:

groovy:
  precise: &pc_arm ['amd64', 'i386', 'armel']
  quantal: *pc_arm
  wheezy: ['armhf']

hydro:
  precise: *pc_arm
  quantal: *pc_arm
Use arches from rosdistro repo

In a later commit: filtering with a command-line parameter
Added an argument to specify the rosdistro repo URI

This is intended to replace the hard-coded
URL_PROTOTYPE="https://raw.github.com/ros/rosdistro/master/releases/%s.yaml"

Will be used in a later commit.
Prepare for some cleaning

functions like doit() and the like take a lot of parameters.
The code could be way cleaner (and more extensible) with
some option grouping. It will also provide some
naming consistency.

Will use in a later commit.
Code cleanup

JobParams and PackageParams are now used to store parameters
among function calls.