Add analogy task evaluation code.

@piskvorky @ogrisel 

This adds code for evaluating the embeddings on the analogy tasks from the word2vec paper. 

It's fairly model-agnostic so should be useful in comparing the performance of `glove` and `gensim`'s `word2vec`. 
If you do this under Python 3, it's best to give the encoding of datafile as additional argument. As it's not possible under Python 2, it better to do:

```python
with open(filename, 'rb') as datafile:
    for bytes_line in datafile:
        text_line = bytes_line.decode('utf-8')
        ...
```