Version should be configurable in plugin

Nobody has claimed this yet.

The plugin currently uses the hard coded `project.version` which may, in development enviroments, not be compliant with [PEP-0440](https://www.python.org/dev/peps/pep-0440/).

Additionally, in the java/maven world, `-SNAPSHOT` is used for versions while python uses `.preview`.

This problem exposes itself when you try to create an egg as I have done in #78.
The egg for the version `master_2016111701-SNAPSHOT` has its version converted to `master_2016111701_SNAPSHOT` (notice the `-` character has become a `_`).

The result is that the egg has the name `$project.name-$project.version.toString().replace("-", "_")` which is very non-obvious.

The fix to this would be to put the `project.version` in the python plugin's properties but allow the user to override this value.

[Example of location where version is hard coded](https://github.com/linkedin/pygradle/blob/master/pygradle-plugin/src/main/groovy/com/linkedin/gradle/python/tasks/SourceDistTask.java#L58)

Start at pygradle-plugin/src/main/groovy/com/linkedin/gradle/python/tasks/SourceDistTask.java around line 58, then trace how the Python plugin exposes its properties. Make the version default to project.version while allowing users to override it, and verify that generated egg names preserve the configured Python-compatible version.

Written by the indexing model from the issue text.