Bulk API fixes

o We didn't manage the index offsetting correctly for bulk inserts.
o We didn't massage errors into writeErrors for legacy operations
cluster: explicitely check for NULL replSet.

Fixes CDRIVER-409.
Merge pull request #65 from hanumantmk/writeErrorFix

Bulk API fixes
allow bulk op insert for w:0 bulk inserts

o fixes a performance regression for 2.6 w:0 writes
o allows w:0 writes to fast path out of the bulk api on 2.4
Add a link to the docs
Merge pull request #67 from bjori/patch-3

Add a link to the docs