Subclass unicode for String and Symbol

Refactoring the String and Symbol classes into subclasses of unicode decouples
from SExpBase. Neither of these classes will creep features. The behavior of
__eq__ has been preserved and doctests were added. Two __sexp__ methods were
used for consistency---registering directly to tosexp() might be faster.
Petrified Quoted and Delimiters, obsoleted SExpBase
more exception coverage
Removed sexpdata import in setup.py to fix singledispatch dependency
Include README.rst in MANIFEST.in for setup.py on Travis
Version-dependent install_requires, Travis fixed (?)

Install ordereddict on 2.6
removed singledispatch from tox setup