[Fix] Set NIST_SIG as character array in order to use sizeof to obtain its length.
[Fix] We were only setting to zero the size of the pointer, not the size of the structure
[build] Make clean faster by avoiding dependency computation
[build] Build shared libraries only if SHARED is not zero
[spelling] alot -> a lot
[Fix] Avoid polluting global namespace in headers.

"using namespace std;" imports all the std namespace to the
current translation unit. We should not impose importing
the std namespace to other developers so we should avoid
using that expression in public headers.

Therefore, all the "using namespace std" declarations have been
moved to internal sources. Explicit calls have been used in
headers instead (i.e. std::cout instead of cout).
[build] Allow using --enable-shared and --enable-debug in ./configure
[build] Allow using --with-nas_audio and --with-pulseaudio in ./configure
[git] Add .gitignore to the repository