added Bradley 2013 GMPE

Implements GMPE developed by Brendan Bradley for Active Shallow Crust
    Earthquakes for New Zealand, and published as "A New Zealand-Specific
    Pseudospectral Acceleration Ground-Motion Prediction Equation for Active
    Shallow Crustal Earthquakes Based on Foreign Models" (2013, Bulletin of
    the Seismological Society of America, Volume 103, No. 3, pages 1801-1822).

    This model is modified from Chiou and Youngs, 2008,2010 and has been adapted
    for New Zealand conditions. Specifically, the modifications are related to:
    1) small magnitude scaling;
    2) scaling of short period ground motion from normal faulting events in
       volcanic crust;
    3) scaling of ground motions on very hard rock sites;
    4) anelastic attenuation in the New Zealand crust;
    5) consideration of the increates anelastic attenuation in the Taupo
       Volcanic Zone (not implemented in this model, use Bradley2013Volc)
Scientifically LGTM.
Given that this GMPE represents a modification of Chiou & Youngs (2008) I am wondering whether it might have been preferable to inherit directly ChiouYoungs2008. Not critical though, so I am happy for this to be merged