Adding native-C BSON support for many of Ruby/Rails' classes that were not currently supported.

These include:

- BigDecimal (converted to BSON Float type 0x01)
- ActiveSupport::TimeWithZone (Converted to BSON 'UTC datetime' type 0x09)
- DateTime (Converted to BSON 'UTC datetime' type 0x09)
- Date (Converted to a BSON UTF8 String type 0x02)

Note: values of "Date" class are converted to a string with USA date format, "mm/dd/yyyy".