rsolve: use limit() (with fallback to subs()) to substitute initial values of n in the generic solution of the recurrence equation
rsolve: simplify handling of the initial conditions
rsolve: tune exception handling for limit(), fallback to subs() in case NotImplementedError
Merge branch 'master' into tmp

Conflicts:
	sympy/solvers/tests/test_recurr.py
test_recurr.py: spaces after comma added
Merge branch 'master' into use-limit-in-rsolve
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