fix unwinding symmetric coroutine
Merge pull request #7 from nat-goodspeed/develop

Documentation review: minor wording changes.
docu update
graphics updated
introduction of stack_traits
update of documentation
docu
symmetric_coroutine::get() throw invalid_result
docu
douc updated
fix typo for protect_stack_allocator on Windows
add missing header attributes.hpp
pass templated coroutine-fn to symmetric_coroutine

- clang-c++11 does not accept templated fucntions to be passed to
  coroutine-ctor (in contrast to gcc-4.8.1)
some code cleanup related to segmented stacks
add clang support for segmented stacks
use traits_type::page_size() in protected-stack_allocator
symmetric-coroutine: placement new + StackAllocator

- for symmetric_coroutine<> StackAllocator is type-erased
- symmetric_coroutine_impl<> is allocated via placement-new on
  coroutine's stack -> no extra jump to coroutine-context
small changes related to symmetric-coroutine
asyymmetric-coroutine with placement new
dd missing keyword typename