Keep http history+1

[sorry for the noise.  updated the last PR to 0.5.4 and rebased.]

**New Feature**: Instead of lastHttpRequest, keep track of all requests and responses so far in WDSession.

**This does not break the API**: lastHttpRequest is now implemented as a one-liner using the new WDSession field.

If you would like me to think more about lazyness in sendHTTPrequest or provide a way to empty wdSessHist conveniently, please let me know.

I guess 0.5.4.0.3 technically maintains package versioning policy, but I'd like to keep the numbers down for sanity. Maybe 0.5.5?