Update year to 2013 everywhere

This also clears trailing whitespace in isympy.xml.
Merge branch '2013'
Merge pull request #1687 from mrocklin/more-branch-strats

Add chain, onaction, and sfilter to branching strategies
unify, rewriterule, and deconstruct take wilds
remove patternify
remove the use of SymPy.Wild in unification
add non-Expr test