Feature/test headers

This adds the test-headers make target which just repeatedly calls the new make target for creating a *.gch (g++) or *.pch (clang++) file from a .hpp file. This entailed renaming a couple .hpp files that were not actually intended to be stand-alone header files to the .h extension.

```
goodrich@CYBERPOWERPC:/opt/stan$ for COMPILER in $(ls /usr/bin/*g++*); do if [[ $COMPILER == *clang* ]]; then make CC=${COMPILER} src/stan/prob/transform.pch 2> /dev/null && echo "clang++-based compiler succeeded"; else make CC=${COMPILER} src/stan/prob/transform.gch 2> /dev/null && echo "g++-based compiler succeeded"; fi; done
/usr/bin/clang++ -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -Wno-unused-function -Wno-uninitialized -O3 -o /dev/null src/stan/prob/transform.hpp
clang++-based compiler succeeded
/usr/bin/clang++-libc++ -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -Wno-unused-function -Wno-uninitialized -O3 -o /dev/null src/stan/prob/transform.hpp
clang++-based compiler succeeded
/usr/bin/g++ -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -Wno-unused-local-typedefs -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/g++-4.4 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/g++-4.5 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/g++-4.6 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/g++-4.7 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/g++-4.8 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -Wno-unused-local-typedefs -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/g++-libc++ -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -Wno-unused-local-typedefs -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/g++-snapshot -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/i686-w64-mingw32-g++ -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -m32 -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/llvm-g++-4.2 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/llvm-g++-4.7 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/x86_64-linux-gnu-g++ -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -Wno-unused-local-typedefs -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/x86_64-linux-gnu-g++-4.4 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/x86_64-linux-gnu-g++-4.5 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/x86_64-linux-gnu-g++-4.6 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/x86_64-linux-gnu-g++-4.7 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/x86_64-linux-gnu-g++-4.8 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -Wno-unused-local-typedefs -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/x86_64-linux-gnu-llvm-g++-4.2 -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
/usr/bin/x86_64-w64-mingw32-g++ -x c++-header -c -I src -I lib/eigen_3.1.3 -I lib/boost_1.53.0 -Wall -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE -DBOOST_DISABLE_ASSERTS -m32 -O3 -o /dev/null src/stan/prob/transform.hpp
g++-based compiler succeeded
goodrich@CYBERPOWERPC:/opt/stan$ 
```