Implementation of the Timeout parameter for querying services

Following pyvirtobs/pyvo issue #10
This commit implements the timeout parameter solution as a global parameter at the dal/query.py level.
The parameter can be (un)set and read through the corresponding functions (unsetparam and setparam); this is a global configuration (again, at the query.py level) that any function in that module can see/use.
Also, as a per-function "solution" a timeout parameter was added to the DALQuery.execute* functions; but here further development/thinking must be done, since higher level objects (e.g, SCSQuery, SIAQuery) are overloading those functions... It not clear to me yet how to deal with this interface "conflict".
Cheers.