Now we can keep the exception information on Python interpreter. This is
usefull if you are working on Python interface. You can catch the right
Exception class when error occurs in your `PythonLayer`. Now we don't
use `PyErr_Print` that clears exceptions. We use `PyErr_Fetch` to get
the information and `PyErr_Restore` to restore the exceptions back. And
I find that `bp::import` or `bp::object::attr` breaks Python interpreter
if a module or an attribute doesn't exist. We now use `bp::exec` and
`bp::eval` to keep Python interpreter working even after errors occur. I
got the hint from the following page.

http://boost.2283326.n4.nabble.com/embedded-python-td2702335.html

And I also modified to use `bp::object` for `self_` member.

* Modified Makefile to link boost_regex for PythonLayer.
* Travis installs boost_regex
* Add a note of a new dependency boost_regex on Makefile.config.example