ENH: add condition count to Epochs __repr__

This in part addresses #338 

I started in a simple way to leave some room for discussion:

```Python

In [10]: epochs
Out[10]: 
<Epochs  |  n_events : 145 (good & bad), visual_r: 73, audio_l: 72, tmin : -0.2
(s), tmax : 0.5 (s), baseline : (None, 0)>

```

Tell my what you think.