Rewrite Sodium::Buffer to use an FFI::Pointer rather than a String

This avoids nastiness like copying garbage collectors from copying our
memory all over, forcing us to lose control of it. More importantly, we
fix a nasty bug related to garbage collection. If you retain a pointer
to a Sodium::Buffer's bytes but not the buffer itself, the buffer can be
garbage collected at any time (clearing your bytes).

The new implementation removes Sodium::Buffer#to_str and replaces it
with Sodium::Buffer#to_ptr and Sodium::Buffer#to_s. to_ptr gives you the
actual memory pointer to do with as you like. to_s gives you the actual
String contents, at which point guarantees are no longer made as to our
ability to clear the memory pointed to by the string. It is the caller's
responsibility never to call to_s on sensitive data unless absolutely
necessary.
Merge pull request #18 from stouset/garbage-collection-safety

Rewrite Sodium::Buffer to use an FFI::Pointer rather than a String