sys: timex cleanup

This PR comprises some improvements for the timex module:
 * better and more documentation
 * some inline functions
 * a more flexible string conversion
I tried to find a website / document on google which contains a similar phrase / wording @OlegHahm used and I didn't find any.
However, the phrase "Calculates the difference between a and b" can be found all over the web.

Therefore, I would be strongly in favor of using the wording (a highly common phrase / easy to understand even for non-native speakers):
"Calculates the difference of two timestamps"

BTW @Kijewski : I know what you mean regarding the word difference. However, 
the result of a subtraction is called a difference by definition. Also, there is such
a thing called "absolute difference" in English so there is no real danger of confusing 
those two.