store secret keys encrypted where possible

The secret spend key is kept encrypted in memory, and
decrypted on the fly when needed.

Both spend and view secret keys are kept encrypted in a JSON
field in the keys file. This avoids leaving the keys in
memory due to being manipulated by the JSON I/O API.

tests/unit_tests/CMakeLists.txt

@@ -27,6 +27,7 @@
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 set(unit_tests_sources
+  account.cpp
   apply_permutation.cpp
   address_from_url.cpp
   ban.cpp