TurtleApp accepts argc by reference

Otherwise running turtlesim_node with a command line argument, such as
`rosrun turtlesim turtlesim_node __name:=sim`
produces a segmentation fault with the following crash report on OSX

<pre>
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   QtCore                        	0x000000010a2c1047 QString::fromLocal8Bit(char const*, int) + 25
1   QtCore                        	0x000000010a358fc2 QCoreApplication::arguments() + 98
2   QtGui                         	0x0000000109793a34 -[QCocoaApplicationDelegate application:openFiles:] + 222
3   com.apple.AppKit              	0x00007fff89907f16 -[NSApplication _doOpenFile:ok:tryTemp:] + 274
4   com.apple.AppKit              	0x00007fff896501ef -[NSApplication finishLaunching] + 1678
5   com.apple.AppKit              	0x00007fff8964f860 -[NSApplication run] + 125
6   QtGui                         	0x000000010979ae11 QEventDispatcherMac::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 461
7   QtCore                        	0x000000010a354d89 QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 79
8   QtCore                        	0x000000010a354ee7 QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) + 341
9   QtCore                        	0x000000010a357a6b QCoreApplication::exec() + 199
10  turtlesim_node                	0x000000010963e238 TurtleApp::exec() + 104
11  turtlesim_node                	0x000000010963de82 main + 66
12  turtlesim_node                	0x000000010963de34 start + 52
</pre>

See discussion at http://qt-project.org/forums/viewthread/18703/