Speed up BSON unmarshalling#410

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craiggwilson wants to merge 3 commits intogo-mgo:v2-unstablefrom craiggwilson:bson_speed

+2,374-377

Commits on Feb 28, 2017

skipping elements/documents when decoding to raw to speed up unmarshalling.
craiggwilson
committed

Commits on Mar 1, 2017

implemented bson_corpus tests and fixed the small number of issues that arose.
craiggwilson
committed

Commits on Mar 2, 2017

fixed issue with skipping binary and added decode only tests for skipping elements.
craiggwilson
committed