Working abroad post possible Brexit

I am a British mature student at a British university with two years left of a four-year course. I have worked in France before and would greatly like to be able to return to work in the EU once graduated. However, the prospect of the UK leaving the EU raises a number of questions.

If the UK was to leave the EU, is it likely that our rights to work in member states would be substantially restricted? And supposing I was able to find employment in the EU before Brexit, would the right to stay and work in that country be then honoured when we come to leave, or merely the contract at the time? This has all become more urgent because the referendum is scheduled for 2016, before I graduate in 2017.