Generalize the code printing of And, Or, Not.

Move the printing of And, Or, and Not up in the class hierarchy to
the CodePrinter class and make it customizable by overriding a dict.
Add Fortran code printing for And, Or, Not, add tests.
Generalize the code printing of Relational.

Allow relational operators to be overriden using a dict.
Fix the Fortran printing of Ne, add tests.
Implement Fortran printing of Xor, Equivalent, add tests.
Fix the precedence order for Relational.

Relationals are somewhere between And and Add operations
in the precedence tables for python, C, Fortran, etc.
Add test for bug in piecewise integration

*  Integral(exp(x*t),(t,0,y)).doit() fails
Fix bug in Piecewise._eval_interval

- now has a consistent return form
Remove the spaces from the definitions of operators.

Instead, the spacing is done directly by the _print_Op methods.
Merge pull request #2556 from jrioux/printing

Fortran code printing of logical and relational operators.
Merge pull request #2557 from zanzibar7/bug_piecewise

Bug piecewise
Add Heiner to authors
Updated email address for Heiner Kirchhoffer.
Updated email address in AUTHORS.
Merge pull request #2562 from HeinerKirchhoffer/certik-author-rebased

Add Heiner to authors and correct email address. Rebased to sympy master.
Fixed to_cnf/to_dnf simplification
Changed parameter name, fixed tests