Change to the treatment of BeastConfig.h
* No longer requires its own compiler include path
* Includes use relative paths to locate the file
* Client applications include the file themselves
* Inclusion of BeastConfig.h can be controlled via preprocessor directive
Add workaround/noexcept.h for visual studio
Add abstract streams module:
* New basic_abstract_ostream template for generic output
* New abstract_ostream, common type alias
* basic_scoped_ostream, RAII output to abstract streams