Get parser working under c++11

This commit reimplements work of Robert Goedman
c++11 cleanup
Updated C++11README
C++11README updated with make test-* results
Removed clang detection

This was an attempt to be able to compile with or without c++11
extensions
Merge branch 'develop' into feature/c++11
Test with g++ on mac

Always have template-depth=1028. Replaced the agrregate initialization
(c++11 only) by ...push_back() in program_grammer_def.hpp.
Added a C++11 flag in makefile

Setting C++11 to true will add -stdlib=libc++ -std=c++11 to the CFLAGS
when OS=mac and CC= clang++.
Merge branch 'develop' into feature/c++11
Merge branch 'feature/c++11' of https://github.com/stan-dev/stan into feature/c++11
Merge branch 'develop' into feature/c++11
Final clean-ups and removal of unneccesary files.

Removed C++11README & my test .stan file.
Removed one more comment
Merge branch 'develop' into feature/c++11
Merge branch 'feature/c++11' of https://github.com/stan-dev/stan into feature/c++11
implement C++11 flag on Linux too
Enable multi_gp in the language.
Add multi_gp to the manual.
Fix inverse in manual.
Fix function signature.