support for buildout environments

fixed build command so ``tinkerer --build`` can also be used when using tinkerer in buildout environment, e.g.:

```cfg
[buildout]
parts = eggs

[eggs]
recipe = zc.recipe.egg
dependent-scripts = true
eggs = tinkerer
```

this requires that sphinx's build_main command is called directly instead of spawing new subprocess (which would require to have sphinx installed globally).