Subject: Relativity, maximum speed, length contraction

Hi, I have been thinking and researching relativity in the past day or so (as 
well as at random times throughout my life) and there are a couple questions 
that I can't find online, or for which explanations I have found are confusing 
or incomplete in some way. First, I don't understand how Einstein or anyone for 
that matter could come up with the idea that light is a maximum speed. How was 
that concept developed intuitively? Also, given the maximum speed, I can 
understand time dilation due to velocity, but time dilation due to gravity seems 
to not come directly from the universal maximum speed. How was time dilation due 
to gravity developed?
Also, I have read about length contraction, and I found that it seems 
contradictory. The contradiction I have come up with is that of a spinning 
object – if an object is spinning with its fringes traveling at the speed of 
light, those fringes should contract, however there is no space for it to 
contract into because the direction of motion isn’t linear. 
I also was wondering if mass-energy depends on frame of reference. Does it?
One last thing, accelerative time dilation affects happen under the influence of 
gravity – do they also happen under the influence of electro-magnetic fields?

Thanks in advance!
~Billy

Re: Relativity, maximum speed, length contraction