Support for Priority attribute in Cookies

Google has submitted an draft memo("A Retention Priority Attribute for HTTP Cookies") to the IETF to add a Priority field to HTTP cookies, see [here](https://docs.google.com/file/d/0B3o1IlTKoADVRllKWGlyWGxIVTg/edit). This feature is already enabled in the Chromium/Chrome browser, see [here](https://code.google.com/p/chromium/issues/detail?id=232693). 

This pull request adds functionality to allow Poco's HTTPCookie to correctly represent cookies with this field and to set and get the value of that field. Without it, cookies with the field are handled incorrectly, see #336. This can cause problems when communicating with Google servers or working on network traffic between Chrome and Google servers and using Poco.

I have used this patch in my own(non-github) network proxy project that uses Poco and have had no problems with it for a while- since around November/December when Chrome was updated to start supporting this cookie field and I came up with a patch to my local Poco source to address issues this caused me.

If you would like, I can improve to add "setPriorityLow(),setPriorityMedium(),setPriorityHigh()" functions, since the draft memo makes it clear what string literals the levels correspond to in a conforming implementation.