XFAIL the test case for this code

The code is correct but due to issues with dummies we can not test that.
use replace_dummy in testing an equation with a Dummy
Improve docstring of harmonic numbers and cross refs between numbers in general
Remove wrong dependencies on the recursive __call__.
Merge pull request #1841 from fitze/mechanicsdoc

Mechanicsdoc
Merge pull request #1842 from skirpichev/use-limit-in-rsolve

rsolve: use limit() to substitute initial values of n in the generic solution of the recurrence equation
Merge pull request #1845 from smichr/logcombine

logcombine modifications
A doctest for rcall.
Remove wrong dependencies on "recursive __call__".
Merge pull request #1859 from hazelnusse/fix_codeprinter

Added necessary import of S from sympy.core
Merge pull request #1855 from raoulb/harmonic_number

Rewrite of harmonic numbers into summation
integration of sympyprt extension into sympy Part 1
fontsize fix

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
Merge pull request #1857 from Krastanov/rcall_notdefault

Remove the default "recursive __call__".
temporary fix for display_formatter problems
Split out implicit multiplication & application
Change docstrings
Rewrite function exponentiation
Fix case in function exponentiation
Add Christopher Dembia to AUTHORS/aboutus. Welcome to SymPy!