provide API to save new GatingSet object

[mikejiang]

Currently most of data structures reside in c++, R object only stores:

1. the pointer to c++ data structure
   2.data Environment that stores ncdfFlowSet and axis.labels

so the serialization routine needs to be implemented in c++ to save the first part.
since c++ does not natively support serialization for complex objects, we will consider using boost Serialization library to do the deep copying of entire gating set class.

[mikejiang]

two R APIs are added to save/load gatingSet to/from disk.

archive(G,dir="output/NHLBI/gs")
saves the GatingSet as two files in the specified folder gs:
1 *. rds file (R object)
2 *.dat file (c++ object that stored in R as external pointer)
They share the same file name with different extensions. In the future, we can think of bundling into one zip file.

G1<-unarchive("output/NHLBI/gs")
reloads the gatingSet from disk.

And reloaded gatingSet has passed the testing on major APIs of flowWorkspace.