Add sharenotify option with python snippets

Add new option to launch an event when a share is accepted: --sharenotify <python_snippet_file.py>

Add two examples of python snippets under sharenotify_snippets/ directory, for instance log.py:
```
global slog
slog = open('/var/log/shares.log', 'a')
def on_share(job_id, worker_name, init_time, dif):
	log.info("Loging share for %s" %worker_name)
	slog.write("[%s] %s %s/%s\n" %(init_time,worker_name,job_id,dif))
	slog.flush()
```

Then it can be included executing: 
```
./mining_proxy.py (...) --sharenotify sharenotify_snippets/log.py
(...)
 INFO proxy share_stats.set_module # Loaded sharenotify snippet sharenotify_snippet/log.py
 (...)
```

cat /var/log/shares.log 
```
[1393952045.03] magicpool.sm2 e20d/32
[1393952241.04] magicpool.sm2 75b/32
[1393952246.28] magicpool.sm2 75b/32
```