Improve build_function for CTarget()

[philbit]

Consistent function signature for C code generated by build_function (plus valid code in more cases), fixes #106

[philbit]

This fixes #106 (i.e. the fourth argument is now a proper scalar both in the C function signature and in the julia function signature), but in the process I noticed that C code generation also suffers from other problems. It's using the regular string representation of julia expressions for code generation which causes problems if C syntax is not similar enough to julia syntax. For example, scalar multiplication a la 4a is not allowed in C, but happens automatically in the string conversion of Expr(:call, :*, 4, :a). Another problem is non-existence of certain operators (e.g., :^).

I addressed the problems I noticed so far with workarounds (still using julia expressions, but modifying them before obtaining their string representation), but in the long run it might be necessary to write a proper C code generation engine. This is more to make build_function usable for more-than-trivial cases at all.

I also added #include <math.h> to the generated C code by default, so special functions like sin and exp can be used.

Tests are included.

[ChrisRackauckas]

Awesome! Yes, I think the last thing needed really is to setup a find-replace dictionary for a lot of Julia->C translations and then I think it's pretty good. Thanks!