Add Timeout management

MailChimp API requiere to manage timeout connection. TimeoutException was not usable.
We have the same code value `-99` than here: https://github.com/rezzza/MailChimpBundle/pull/5/files#diff-4c46f9a32fedb86a097be062eedc7ccbR113 Not that handy to distinguish the error origin.

Furthermore could be cool maybe to adopt constants for the codes instead of hardcoded negative int values.