Brevity 2 require iOS 8 to function. If you're on iOS 7, check out the original Brevity (iOS 7) on Cydia!
Brevity allows you to respond to text messages quickly and easily. You set your desired shortcut, and when nothing's typed, it takes the place of the microphone button in iMessage, and the greyed-out send button in Text Messages. With one touch, your shortcut's sent. If you want to send a voice message, simple hold on the brevity icon just as you would normally.
When you want to send a standard message, just go ahead and start typing. The send button fades right back in, and you're ready to go.
Your shortcut can be anything, from plain text to emoji, or even unicode characters. Changing your shortcut takes one quick trip to a preferences pane, and does not require a respring.