Allow a value of 0 with the --intra / -g option, which is handled by the codec as only send an I-frame on the first frame and never again.
Update raspivid help and README.md explaining the use of 0 with the --intra / -g option.
Merge pull request #185 from zboobzor/master

Allow a value of 0 with the --intra / -g option, which is handled by the...