Simplify base class detection for Eigen types

Add in casts for c++11s chrono classes to pythons datetime

Make the accessor interface more complete

`auto var = l[0]` has a strange quirk: `var` is actually an accessor and
not an object, so any later assignment of `var = ...` would modify l[0]
instead of `var`. This is surprising compared to the non-auto assignment
`py::object var = l[0]; var = ...`.

By overloading `operator=` on lvalue/rvalue, the expected behavior is
restored even for `auto` variables.

This also adds the `hasattr` and `getattr` functions which are needed
with the new attribute behavior. The new functions behave exactly like
their Python counterparts.

Similarly `object` gets a `contains` method which calls `__contains__`,
i.e. it's the same as the `in` keyword in Python.

Use consistent indentation and typenames in numpy vectorize. 

Fix minor documentation spelling mistakes

Fix missing smart_ptr test

WIP: Multiple inheritance support

The custom exception handling added in PR #273 is robust, but is overly
complex for declaring the most common simple C++ -> Python exception
mapping that needs only to copy `what()`.  This add a simpler
`py::register_exception<CppExp>(module, "PyExp");` function that greatly
simplifies the common basic case of translation of a simple CppException
into a simple PythonException, while not removing the more advanced
capabilities of defining custom exception handlers.

Fix spaces before parens for style guide.

Allowed durations and non system clocks to be set from floats.

Add unit tests and documentation for the chrono cast.

Debug build and fix

Take load_type by nested type_caster template arguments instead of by
full type_caster type.