Add include guards to cxxopts.hpp
untangle includes

- order lexicographically
- include all used headers directly instead of implicit includes
- remove unused header
- move regex include into compile unit
Merge pull request #1 from wichtounet/patch-1

Add include guards to cxxopts.hpp
Merge branch 'untangle_includes' of https://github.com/DennisOSRM/cxxopts into untangle-includes

Conflicts:
	src/cxxopts.hpp
Merge branch 'untangle-includes'
Some minor code polishing:

- const as const can
- use C++11'ism where possible