Add operators to Timestamp for Timespan.

Timestamp  operator +  (const Timespan& span) const;
Timestamp  operator -  (const Timespan& span) const;
Timestamp& operator += (const Timespan& span);
Timestamp& operator -= (const Timespan& span);
Merge pull request #180 from syvex/TimestampTimespanOperators

Add operators to Timestamp for Timespan.
Merge https://github.com/pocoproject/poco into develop
SharedPtr: made constructor explicit, disallow implicit casting to raw ptr