Rename getters functions removing 'get_' prefix

Follow the suggested Qt style:

http://doc.qt.nokia.com/qq/qq13-apis.html

It seems to me simpler and easier to read.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/book.cpp

@@ -349,7 +349,7 @@ namespace {
 /// Book c'tor. Make random number generation less deterministic, for book moves
 Book::Book() : bookSize(0) {
 
-  for (int i = abs(get_system_time() % 10000); i > 0; i--)
+  for (int i = abs(system_time() % 10000); i > 0; i--)
       RKiss.rand<unsigned>();
 }