Added an assert with an extra message

This pull request is only for my commits afc606b, eed270b, 26245be and 2a2fc30. Cant figure out how to re-use the same branch, but remove other's commit from my pull requests. :)

An example of using the new macro:

poco_massert(tt > vec.back().timestamp, "Transactions must be added in chronological order!");

An example of using the Min/Max - findiing the minimum timestamp in a vector of timestamps (timestampVector):

Timestamp mm(Timestamp::Max);
for(tt : timestampVector)
   if(tt < mm) mm = tt;