Adding gyp build file and including string.h in base64.h for memset.
Merged in many of the changes from https://github.com/kloplop321/bson-cpp.git
to remove most boost dependencies except for the boost time library used in 
util/time_support.h and boost spirit in json.cpp. The latest mongodb version
has removed boost depencies so will have to merge in those changes later.

Disabled thread safety for boost spirit to avoid linking libboost_thread.