Add comment about dependents on the internal structure of Calculus.jl

Other packages starts to depend on the internal structure of this
package. I think it is reasonable to add a comment to tell future
developers that might want to change the internal structure which public
packages they will break, so that they can ping the authors.
See: https://github.com/scidom/DualNumbers.jl/pull/1
Merge pull request #35 from ivarne/patch-1

Add comment about dependents on the internal structure of Calculus.jl
Add factor of pi/180 to degree versions of trigonometric functions. Fix #37
Merge pull request #38 from magistere/trig

Add factor of pi/180 to degree versions of trigonometric functions. Fix #37
Using optional arguments in integrate.jl
Cleanup in differentiate.jl
More cleanup. Change array type from Expr to Any since differentiate can also return Numbers