Merge kmillar/cxxr/cxxr-jit into cxxr-devel/cxxr/master

Build changes:
  - Requires the compiler to support C++11 (previously was C++98)
  - Adds a '--enable-llvm-jit' option to configure.
  - '--enable-llvm-jit' requires LLVM 3.4.  The path to LLVM can be specified with '--with-llvm'.  e.g. '--with-llvm=/usr/bin/llvm-config-3.4'
  - 

Major functionality changes:
  - Adds a LLVM-based JIT compiler.  At this point, the compiler can inline most control flow operations ('for', '&&', '||' are yet to be implemented) and can lookup local variables efficiently.  All other functionality is passed on to the interpreter.

Minor changes:
  - A number of bugfixes and portability improvements.