ENH: added tic, tac, toc functions

I always lack the tic, toc functions from matlab. Here is a trivial implementation. It works exactly like Matlab's version, has an additional tac() function.
Here is an example:

```
from quantecon.timing import tic,tac,toc

tic()  # initializes timer
for i in range(10):
    sleep(1)
    tac()   # prints: "Elapsed: 1 seconds."
toc() # prints: "Elapsed: 10 seconds."
```