Make the FDW work in PostgreSQL 9.2

That is a simple solution to make the mongo_fdw work on PostgreSQL 9.2.
There is no performance improvements that the 9.2 version provided, like
better estimate plans. Basically we created the MongoGetForeignRelSize
function based on old MongoPlanForeignScan (encapsulating the calls on
MongoGeneratePlanState function), and basic (dummy) MongoGetForeignPaths
and MongoGetForeignPlan functions (based on file_fdw).
Add support for oid timestamps by suffixing column with .generated
Make sure to multiply long by a long.
Add debug for timestamp conversion.
Add debug statement.
Set time_t to 0 to prevent garbage in first 4 bytes.
Cast bson_oid_generated_time to int32 to work around bson.c bug.
Add a ton of type coercion.
Two bug fixes.  One, ignore field option if it is an empty string.  Two, properly convert ints to timestamps.
Merge in basic 9.2 changes from https://github.com/matheusoliveira/mongo_fdw/commit/004e58bc5b146f88814afa5b39259aee02cc3fa6
Add column mongo_type option to help push queries down correctly.
Fix bug when array comes back null in bson.