Demand for Ruby, Hadoop and HTML5 developers jumped this year, with jobs requiring those skills increasing 70 per cent compared to the same period in 2010, according to a survey of the tech jobs in London by recruiters Azuna. Azuna collated every tech job advertised for London last month, a total of 100,000. HTML coders are still the most in demand, but also the most poorly paid – both at entry and top levels.…