Add another missing mutable.
Document behaviour of join().
Ensure shutdown_context is idempotent.
Partial implementation of executor_wrapper typedefs.

Implemented argument_type, first_argument_type and second_argument_type
for plain function pointers and references.
Add pipeline example.
Remove unnecessary joinable check.
Merge remote-tracking branch 'upstream/master'
More implementation of executor_wrapper typedefs.

Implemented argument_type, first_argument_type and second_argument_type
for types that define these nested typedefs themselves.
Workaround Xcode's incomplete reference_wrapper implementation.
Fix non-boost build.
Add missing headers to makefiles.
executors version 0.1.0
Merge remote-tracking branch 'upstream/master'