add Copr releaser

based on KojiReleaser, with few minor changes in KojiReleaser to prevent
duplication of code.
To learn more about Copr see:
https://fedorahosted.org/copr/
section for project in tito.props is not mandatory for CoprReleaser

so if it does not exist, create it. This is in-memory operation
and does not affect real file.
Also make disttag optional. If not defined use empty string.

addressing:
Building release in Copr...
Traceback (most recent call last):
  File "/usr/bin/tito", line 23, in <module>
    CLI().main(sys.argv[1:])
  File "/usr/lib/python2.7/site-packages/tito/cli.py", line 94, in main
    return module.main(argv)
  File "/usr/lib/python2.7/site-packages/tito/cli.py", line 639, in main
    scratch=self.options.scratch)
  File "/usr/lib/python2.7/site-packages/tito/release.py", line 998, in release
    self._koji_release()
  File "/usr/lib/python2.7/site-packages/tito/copr.py", line 45, in _koji_release
    KojiReleaser._koji_release(self)
  File "/usr/lib/python2.7/site-packages/tito/release.py", line 1029, in _koji_release
    disttag = self.builder.config.get(koji_tag, "disttag")
  File "/usr/lib64/python2.7/ConfigParser.py", line 607, in get
    raise NoSectionError(section)
ConfigParser.NoSectionError: No section: 'copr'
Merge the FiledVersionTagger into the base VersionTagger.

This allows the behaviour to be used in just about any tagger sub-class.

Behaviour will be triggered on the presence of the 'version_template'
section in tito.props.

Renamed config:
  version -> version_template (section)
  file -> destination_file

Separated the replacement of version and release, as caller may just
want one or the other. (it works fine if only one of the variables is
present in your template file) Release is more of an rpm concept so may
not always be wanted in this context.

Removed the notion of default template files, if you want to use this
you need to specify the configuration for it.

Added a functional test to make sure I keep it working.
Merge pull request #97 from xsuchy/pull-req-copr-releaser2

add Copr Releaser
Automatic commit of package [tito] release [0.4.18-1].