add ability to write environment properties

This line is somehow not covered with tests (https://travis-ci.org/allure-framework/allure-python/jobs/40582964):
```
py27 runtests: commands[1] | coverage report -m --fail-under=96
Name                   Stmts   Miss  Cover   Missing
----------------------------------------------------
allure/__init__            4      0   100%   
allure/common            103      3    97%   25-27
allure/constants          33      0   100%   
allure/pytest_plugin     155      3    98%   211, 233, 346
```

How is that so?