Added optimization in Elemwise C code generation.

Elemwise now works on simple case generating only one C file. I'll add the dtype declarations for differents dtypes input in a later commit.
Forget the above comment. I'll delete it. Here is the good one:

checkNDim and checkType are always empty string.
I think you should call c_check() on set(inames). This will duplicate dtype check on inames[0], but doing c_checkType and c_checkNdim don't do all checks. Hopefully the compiler can remove the duplicate check on inames[0] dtype.