Bugfix MySQL NULL and add numeric & date/time datatypes.

[golddranks]

Bugfix

• The MySQL data type conversion failed when requesting a NULL value.
  • This was because one can request the desired type from mysql_common library as a generic parameter, and it considers MySQL NULL types to be convertible only when the requested type is Option<T>.
  • Confusingly, the outer Option seems to signify whether the value is taken from the Row, it has no connection to the nullability of the type.

Additions

• Added MySQL datatypes for numeric types
• Added MySQL datatypes for date/time types
• These additions might need to be reviewed carefully, since I'm not exactly sure whether the types match for what's intended for the rdbc::DataType type. (Ref: Datatype for 64-bit integers (long long)? #42 )

Chores

• format!("{:?}", e) -> e.to_string(), seems to be simpler and avoids the format machinery where it's not needed.

[andygrove]

LGTM!