Implements the Boore, Joyner & Fumal (1997) GMPE

Implements the Boore et al (1997) GMPE because ... well I thought it might cheer me up.

The original Boore et al (1997) GMPE provides coefficients to allow for prediction of the geometric mean horizontal component and for the arbitrary horizontal component. I therefore create two separate classes, the original for geometric mean and a subclass for the horizontal component.

Furthermore, the "Boore" family of GMPEs allow for an unspecified style-of-faulting mechanism. To accommodate this I subclass both classes to allow implementation that neglects the rake of the rupture.

1) Implements four classes (BooreEtAl1997, BooreEtAl1997Unspecified, BooreEtAl1997Arbitrary, BooreEtAl1997ArbitraryUnspecified)

2) Tests. The implementation by Baker is used as a reference (http://www.stanford.edu/~bakerjw/GMPEs.html)

3) Updates the docs