Created a property for bytesPerSecond and changed the way videoBitRate is set

Added the bytesPerSecond property in the .h file. 
Set bytesPerSecond in the .m such that when it is set, it also makes sure that the videoBitRate changes to “bytesPerSecond * 8.” 
Changed the suggested bytesPerSecond defined in the .m file to an ENUM so that someone can call the descriptive name (PBJBytesPerSecond480X360) rather than a number (87500).

These make it easier to use in code (one doesn't have to remember to multiple by 8, reduces errors with copying numbers, makes code easier to read by descriptive names, etc).