Implement a new syntax for multivariable Order symbol

    Supported input:
    O(expr)
    O(expr, (var1, pt1), (var2, pt2), ...)
    O(expr, var1, var2, ...) == O(expr, (var1, 0), (var2, 0), ...)
Fix Order syntax in some tests
Fix tests for as_expr_variables method
Fix printing tests
Test that Order(x, 2 - x) raises now TypeError, not NotImplementedError
Test that variables are unique symbols
Test that O(O(x), (x, oo)) raises an exception
O(O(expr, vp)) -> O(expr, vp)
Add Order.variables to the return value of Order.free_symbols
Delete ancient type cast
Add note: all limit points should be either 0 or oo
Make definition consistent: \lim -> \lim\sup
Add examples
Merge branch 'master' into order-fix-syntax2
Add missing imports for docstring examples
Merge pull request #2613 from skirpichev/order-fix-syntax2

Implement a new syntax for multivariable Order symbol (v2)