ENH: add `qiime2.__release__` API (#210)
ENH: add `qiime2.__release__` API (#210)

`qiime2.__release__` is a string containing the "train release" in `<year>.<month>` format. It is `__version__` with patch and pre/post-release tags stripped.

For example:

```
__version__ = '2017.2.0.dev0'
__release__ = '2017.2'
```
thermokarst/qiime2,qiime2/qiime2,biocore/qiime2,jairideout/qiime2,thermokarst/qiime2,ebolyen/qiime2,biocore/qiime2,jakereps/qiime2,nervous-laughter/qiime2,qiime2/qiime2,jakereps/qiime2