Why can't shutterspeed be set?

[calocedrus]

This is a great library, but I need to be able to capture at high frame rates (> 120fps), and currently the shutterspeed can't be set.
I've captured video at such framerates with the default raspivid (from raspbian), so why can't the shutterseep in raspicam be set?
What has to be done to be able to use this property?

Reference:
Shutterspeed is currently set to automatic. The setter is never used, nor can be used. This implicates that the getter will always return -1

Originally posted by @cedricve in #2 (comment)