Squashed commit of the following:
  add libprimecount, implementations of nextprime
  and related things

 Changes to be committed:
	modified:   README.md
	deleted:    TODO.md
	new file:   clean.sh
	modified:   deps/.gitignore
	modified:   deps/build.jl
	new file:   deps/src/cprimecount/.gitignore
	new file:   deps/src/cprimecount/Makefile
	new file:   deps/src/cprimecount/cprimecount.cpp
	modified:   src/PrimeSieve.jl
	new file:   src/compatibility.jl
	modified:   src/deepconvert.jl
	new file:   src/nextprime.jl
	new file:   src/primecount_c.jl
	modified:   src/primesieve_c.jl
	new file:   src/tuples.jl
	modified:   src/wrappers.jl
	new file:   test/primecount_test.jl
	renamed:    test/prime_test.jl -> test/primesieve_test.jl
	modified:   test/runtests.jl
	new file:   test/tuples_test.jl