Added support for a NULL realloc function to bson_writer
_bson_grow errors were being ignore by _bson_append_va. Modified _bson_append_va to propogate _bson_grow errors
Added BSON_UNLIKELY to the _bson_grow failure case
Merge pull request #18 from jjwchoy/null-realloc

Added support for a NULL realloc function to bson_writer.