Replace delete-backward-char with delete-char

Fixes several compilation warnings (#105)
Use string-to-number instead of the obsolete string-to-int
Enable travis-CI build job for haskell-mode

For now this only checks whether the `.el` files compile at all

Some in the future we can turn some of the warnings into errors, to
make the jobs fail...
Fix minor typo in README.md
Add Travis-CI build-status image to README.md
Replace functions used only interactively
Replace interactive-p with called-interactively-p

If support for Emacs versions prior to 23.1 is to be dropped (#156) than
interactive-p (obsolete as of 23.2) can be removed.