Fix ticket #2700: some numeric IDs were misinterpreted as hex numbers instead of...

Fix ticket #2700: some numeric IDs were misinterpreted as hex numbers instead of strings when '0x123' passed in.

Switched from is_numeric() to a custom self::is_decimal() which is more strict.
This makes our behavior match Twitter's API a bit better, so eg this:

  http://identi.ca/api/statuses/home_timeline/0x6d686b.xml

should now be equivalent to:

  http://identi.ca/api/statuses/home_timeline.xml?screen_name=0x6d686b

instead of:

  http://identi.ca/api/statuses/home_timeline.xml?user_id=7170155