Add time offset into getpeerinfo output

This code extends getpeerinfo output with time offset of the node. The getpeerinfo help text is extended to describe the new variable.
Second commit brings this new info to the UI. The Debug window/Peers tab then looks like this (the new part is marked with purple arrow):
![q](https://cloud.githubusercontent.com/assets/6848764/5435964/3289b928-8461-11e4-88a7-e3b4d287177d.png)
This helps to debug the nodes in the network and their time.