SDSL 2.1

Most important changes to previous release:
[x] sd_vector can now be constructed from an increasing sequence of integers
[x] Port to Windows (compiles with the latest version of VC++)
[x] Tests use CMake
[x] Added a Full-Compressed Suffix Tree (cst_fully)
[x] Added an alphabet partitioned WT (wt_ap.hpp)
[x] Added the hybrid_bitvector (hyp_vector.hpp) of Kärkkäinen, Kempa, and Puglisi
[x] External dependencies libdivsufsort and gtest are now included as submodules