Compile with -std=c++1y.

I am uncertain whether or not this is compatible with clang 3.3 and gcc 4.8,
and cannot test it at this moment, but it should be.
Enable compilation for C++1y.

Add defines to concepts/basic.h to enable conditional compilation.

Test this feature by implementing a variable template version of aPure<F>.
Add mempty_v<M> = monoid<M>::id.

Include basic.h in prelude to pull in the FTL_HAS_* macros.