Subcontext fixes and formatting
Moved is_parser trait to its own file
Added type-erased parser, tests and examples
Merge branch 'develop'

Conflicts:
	include/boost/spirit/home/x3/core/parser.hpp
Merge branch 'origin/x3-devel'
Merge pull request #13 from jamboree/develop

x3 new facade-based API
Merge branch 'develop' into x3-devel
fix transform_params/ fix integer_sequence for clang
Merge pull request #15 from jamboree/develop

fix transform_params/ fix integer_sequence for clang
Fixed const issue in attr_parser
Fixes for boost::optional explicit operator bool
Merge branch 'develop' into x3-devel
Merge branch 'x3-devel' into ex3

Conflicts:
	include/boost/spirit/home/x3/auxiliary/attr.hpp
	include/boost/spirit/home/x3/core/caller.hpp
	include/boost/spirit/home/x3/core/detail/transform_params.hpp
	include/boost/spirit/home/x3/core/directive.hpp
	include/boost/spirit/home/x3/core/parser.hpp
	include/boost/spirit/home/x3/support/utility/is_callable.hpp