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.
if you extend the method signature of '_compute_mean_on_rock' in the 'Asc' class to accept also the hypocentral depth (Hc) you can avoid rewriting the 'get_mean_and_stddevs' method and just overwrite here the '_compute_mean_on_rock' method with the equation specific to the subduction interface.

You will have to generalize the 'get_mean_and_stddevs' method to also deal with the case in which the rake angle is not available (the subduction interface case) and the hypocentral depth is not available (the 'asc' case).

One approach could be to implement in the 'Asc' GMPE two private methods: '_get_rake(self, rup)' and '_get_hypo_depth(self, rup)'. In the 'Asc' GMPE these two methods would return rup.rake and None, respectively. While for the 'Sinter' case you can overwrite them so that they would return None and rup.hypo_depth, respectively.

You can then extend the method: '_get_fault_mechanism_flags' so that if rake is None, just return 0, 0 for CN, CR (anyhow this information would not be used for the mean calculation in the 'sinter' case)