lxc container support

- implement support to build an image that runs as lxc container
  + add "lxc" as accepted value to image attribute
  + add container attribute to <type> element, this attribute is required
    for container building
  + create consistency check for container type
    + must have container attribute
    + container attribute value must be composed of word characters
    + container build must include lxc packsge (runtime check)
    + unit tests
  + implement processing of new attributes in the XML code
    + unit tests
- implement KIWIContainerConfigWriter
  + this class writes the configuration file and fstab file for a container
  + it is a child of KIWIConfigWriter
  + unit tests
- implement KIWIConfigWriter
  + base class for configuration file writers, at present it only has the
    container writer as a child. This class will help us to separate the
    code for other image types that have configuration file (ovf, vmware, xen)
    and create individual classes that are focused on only one thing
- implement KIWIConfigWriterFactory
  + factory class that hands out a *ConfigWriter objects based on the image
    type or image format being built
  + unit tests
- implement KIWIContainerBuilder
  + this class is responsible for the image creation for a container image
    only createImage is public and implements the creation procedure
  + is a child of KIWIImageBuilder
  + unit tests
- implement KIWIImageBuilder
  + base class for all *Builder classes, at present it only has the
    container builder as a child. This class will help us to peal apart
    the creation of various image types currently implemented in a tangle
    in KIWIImage and KIWIImageCreator
- implement KIWIImageBuildFactory
  + factory class that hands out *Builder objects based on the image
    type
  + unit tests
- integrate the new build infrastructure code into the image creation
  process for lxc containers
- add directory exist method to test case
- create container example for openSUSE 12.3 and 12.2
- examples
  + fix the text in the 12.3 examples to refer to 12.3, not 12.2
  + all network configurations, 12.2 and 13.3, STARTMODE value "onboot" is
    no longer supported, changed to "auto"
- add a chapter to the documentation providing information about the
  new container image type