Code to fetch consequences for Structural Variants

This code will step in if the user enters alleles
that look like an SV e.g. DUP, DEL. It creates
StructuralVariationFeature objects in place of
VariationFeature objects, and tests various getter
methods using $obj->can before using them to avoid
e.g. trying to fetch sequence for an SV consequence.
fixes for hgvs and _add_fields method
only get alternate alleles
fixes for motif feature consequences
add SV example to vep
add SV example to vep.conf
add SV example to ensembl_rest.conf
Merge pull request #1 from willmclaren/master

Support Structural Variants on VEP endpoint