escaping unsupported object while printing

Method PRINT-DATA on not specialized types converts theses types to
strings (using princ-to-string) and uses PRINT-DATA on theses strings.

This allows printing of complex types (like hash-tables, objects, etc)
without a readable string representation to be escape printed.

Useful for debugging.
Add support for printing symbols like strings
adds tests for printing unprintable objects and symbols
support sections bound to strings

See https://github.com/kanru/cl-mustache/issues/10 and
https://gist.github.com/zickzackv/8247170
Merge pull request #11 from zickzackv/string-sections

Support sections bound to strings. Fix regression from bf8d437.
Use stricter type specifier.
Use &#39; instead of &apos; to escape #\'. Fixes #12
Fix test case for escaping.
fix escape unreadable objects test

made escape unreadable objects test potable

renamed unprintable-class to unreadable-class
Merge pull request #9 from zickzackv/escape-unprintables

Escape unprintables