Skip cache

Sometimes something goes wrong (e.g. communication with a server times out), and your function wants to return a "fail gracefully" fallback value, but you don't want it cached. The SkipCache exception lets you do just that.

instead of doing:
```python
  return some_fallback_value
```
you do:
```python
  raise SkipCache,"Server timeout",some_fallback_value
```
The caller of your function will get the value, but it won't get cached.
The error text ("Server timeout") will appear in functioncache's log.

"Real life" example:
```python
        try:
            user['avatar'] = self.twister.dhtget(username,'avatar','s')[0]['p']['v'] # yes. it's a "real life" example :)
        except: # Probably a temporary network glitch. Better not cache
            user['avatar'] = None # Client will show the default avatar
            raise SkipCache("couldn't get avatar for @{0}, not caching".format(username),user)
        return user
```