Merge pull request #211 from mpetri/master

Remove googletest sources from repository and use submodule instead
Fix for issue #210

char2comp now checks if the character is in the alphabet.
The std:: namespace prefix was also added in the io.hpp
for stoull (see issue #212).
Merge pull request #213 from simongog/fix_210

Fix for issue #210
Fixed space complexity for bit_vector_il.

Thanks to @diegocaro.
add iterator constructor for sd vector
added test for sd itr ctr