Merge pull request #15 from jamboree/develop

fix transform_params/ fix integer_sequence for clang (reverted from commit 29fea725763e27067f5bdda172d790bd0bda5623)
new facade-based API (reverted from commit 7f7dbe90368a18d977e2c8aea313b0f1e57e016b)
Merge branch 'develop'
removing non-variadics code
ading is_rule trait
better x3::variant interface
add utilities
add core/caller
adapt core/parser
add core/directive
add explicit include/style change
Merge branch 'x3-devel' of github.com:boostorg/spirit into x3-devel
Merge branch 'x3-devel' into ex3

Conflicts:
	include/boost/spirit/home/x3/core/action.hpp
	include/boost/spirit/home/x3/nonterminal/grammar.hpp
	include/boost/spirit/home/x3/nonterminal/rule.hpp
	include/boost/spirit/home/x3/support/utility/is_callable.hpp