tf2: Fixes linkedit errors on OS X
tf2: Fixes compiler warnings on OS X

This exploited a gcc specific extension and is not
C++ standard compliant. There used to be a "fix"
for OS X which no longer applies. I think it is ok
to use this as an int instead of a double, but
another way to fix it would be to use a define.
tf2: Fixes a warning on OS X, but generally safer

Replaces the use of pointers with shared_ptrs,
this allows the polymorphism and makes it so that
the compiler doesn't yell at us about calling
delete on a class with a public non-virtual
destructor.
tf2_py: Fixes warning, implicit conversion of NULL
Merge pull request #11 from ros/fix_osx

Fix linkedit errors and compiler warnings on OS X