[Graph]: trace the reference of each FlashGraphR object.
[Graph]: release test the SAFS mode of FlashGraphR.
[Graph]: add R_FG_set_conf in FlashGraphR.
[Graph]: delete the in-mem graph if it's not referenced in R.
[Graph]: release test if two graphs are loaded with the same name.
[R]: move FlashGraphR to the top directory.
[SAFS]: fix a bug: a SAFS file can't be opened multiple times.
[Graph]: use default compiler to compile tools.
[SAFS]: add namespace safs.
[Graph]: using SAFS with namespace.
[R]: using namespace safs.
[Common]: separate slab_allocator from safs completely.
[Graph]: move bitmap implementation to libcommon.
[Graph]: add namespace fg.
[R]: use namespace fg.
[SAFS]: fix a bug in destroy_io_system.

destroy_io_system only destroys the I/O system when it is invoked the
same number of times that init_io_system is executed successfully.
[Graph]: count init_io_system correctly.
[Graph]: fix ASE for directed graphs.
[Graph]: fix a bug in graph engine initialization.
[R]: fix printing.