__bson_uint32_swap_slow should not use long values
fix implementation and use of bson_strerror_r

o we weren't correctly returning a possibly static string from
  bson_strerror_r.

o we're supposed to use the return value from strerror_r, not just
  assume that the passed in buf was filled
Merge pull request #94 from jeromelebel/__bson_uint32_swap_slow_no_long

__bson_uint32_swap_slow should not use long values
Merge pull request #96 from jeromelebel/strerror_r_returns_0_for_success

strerror_r 0 for success
Merge branch 'mongodb-master'

Conflicts:
	src/bson/bson-error.c
	src/bson/bson-json.c
	src/bson/bson-reader.c