Implemented getVersion API call #729

Implementation of feature request https://github.com/Bitmessage/PyBitmessage/issues/729: API to get current pybitmessage version number.

command: getVersion
parameters: n/a
description: Returns the version string.

The `getVersion` call is important, as it allows API users to determine which operations are available.

Example usage

```python
import xmlrpclib
import json
import time

api = xmlrpclib.ServerProxy("http://user:pass@127.0.0.1:8442/")
print api.getVersion()
```

Note: Upon merging this commit, the [API reference documentation](https://bitmessage.org/wiki/API_Reference) should also be updated.