Add centered forms for scalar functions

[koehlerson]

Identical PR to #25
This time I copied the changes of origin/centered_forms into a local branch centered_forms which was based on origin/master All tests passed locally.

[dpsanders]

Thanks!
Could you please update .travis.yml and appveyor.yml if necessary to remove 0.7 and test on 1.5 instead?

[koehlerson]

CI is up to date now. However, there is one failing test for Appveyor in nightly related to:
https://github.com/JuliaIntervals/SetRounding.jl/blob/1e97962241f6478af4d2cda483644cf265b710e7/src/SetRounding.jl#L45
In Travis everything works