Option to set intra refresh period.

HTTP Live streaming (HDS/HLS/Dash/Smooth) need fixed length GoPs (Group
of Pictures). This option allows the key frame period, and hence GoP
duration to be set using the -g or -intra option. The unit is a number
of frames.

```
  $ raspivid -g 200 -w 1280 -h 720 -fps 25 -t 60000 -b 3000000 -o capture.h264
```
Merge pull request #44 from AndyA/intraperiod

Option to set intra refresh period.