Add mongo_write_concern_create and mongo_write_concern_dispose, consistent with pattern for other types
check all socket reads and return value on error

CDRIVER-199 Socket reads not checked for IO errors, e.g. timeouts
CDRIVER-200 Memory scribble in 'bson_append' when level of nested objects exceeds 32
CDRIVER-200 _bson_append_grow_stack now doubles stack size, initially uses struct-local fixed-sized stack to avoid initial malloc
Merge pull request #84 from jbrd/master

CDRIVER-200 Memory scribble in 'bson_append' when level of nested objects exceeds 32
Slight readability improvement in docs
Merge pull request #86 from geoghegan/master

Slight readability improvement in docs
Add static helper _bson_position(.)
Merge pull request #88 from noa--/bson_position

Add static helper _bson_position(.)
Make consistent use of helper functions to create bson and cursor objects
Also make consistent use of bson_dispose
Update comments and reorder functions to clarify usage of bson_init* and bson_destroy, including the special case of bson_init_data
Merge pull request #77 from noa--/pullreq8

Add mongo_write_concern_create and mongo_write_concern_dispose, consistent with pattern for other types
Merge pull request #89 from noa--/bson_create

Make consistent use of helper functions to create bson and cursor objects
Merge pull request #90 from noa--/memory_usage_comments

Update comments re memory ownership
Clear up a confusing typo
Merge pull request #91 from noa--/replica_set_comment_typo

Clear up a confusing typo
merge from master-mongo-c-driver-master
I guess no more NULL_BSON