Fragility curves

Extend the hazard viewer so that it will render uniform hazard spectra curves

LP: https://bugs.launchpad.net/oq-platform/+bug/1281105

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This PR allows the hazard viewer application (in need of a new, more general name) to plot multiple loss curves to a D3.js chart in a dynamic way. https://bugs.launchpad.net/oq-platform/+bug/1283111