added NZ specific McVerry et al 2006 GMPE for Asc, SInter, SSlab and Volc

Added implementation of New Zealand specific GMPE McVerry et al 2006 for TRT Active Shallow Crust, Subduction Interface, Subduction Intraslab and Volcanic. Based on the paper:

G. McVerry, J. Zhao, N.A. Abrahamson, P. Somerville published as "New Zealand Acceleration Response Spectrum Attenuation Relations for Crustal and Subduction Zone Earthquakes", Bulletin of the New Zealand Society for Earthquake Engineering, v.39, no. 1, p. 1-58, March 2006.

URL: http://www.nzsee.org.nz/db/Bulletin/Archive/39(1)0001.pdf
Last accessed 10 September 2014.
To me the method look a bit too long and therefore it is not easy to read.

I see that the logic:

_compute_mean_on_rock
_compute_nonlinear_soil_term

is repeated three times and the only change is in the coefficients. Maybe you can put this logic into a separate method and call it three times inside 'get_mean_and_stddevs'