Release 1.2.0

Changes since 1.1.0:

* Add a `--commit` option to `git imerge reparent`, which lets a
  sequence of merges be reparented.

* Add three more goal options: `border`, `border-with-history`, and
  `border-with-history2`.

* Retain author information in more situations (i.e., whenever
  retaining the commit message).

* Set up to release to PyPI.

* Switch tests to use `tox`.

* Make Python 3 more prominent than Python 2 in the docs.

* Add axes and labels to the text version of the `git-imerge diagram` output.

* Improve the test suite.

* Convert the README and TODO files to markdown.

Release 1.1.0

Includes:

* Don't be so picky about which branches can be merged together.

* Avoid storing the name of the default imerge in the Git
  configuration when it is unambiguous. This is mostly nice because
  Git leaves section headers behind when configuration sections are
  deleted, so using `git-imerge` used to make the config file untidy.

* Abstract out the interface to Git. This is only an internal
  refactoring, but will hopefully make it easier to add tests and/or
  implement some functionality using libgit2 rather than the Git
  command line.

* Various bug minor fixes.

Merge pull request #104 from mhagger/bash-complete-drop-revert

Add bash completion for new `drop` and `revert` subcommands

Test release v0.7.0.

Why "0.7.0"?  Because git-imerge feels about 70% of the way to a 1.0
release.