Adding function conj

Here is my previous patch in a PR form. The tests pass (complaining conj is not implemented for {float, double, complex..}, but

numexpr.evaluate('conj(a)')
Raises:
NotImplementedError: couldn't find matching opcode for 'conjugate_cc'

I am not familiar with the internals, and I am surely missing something very simple. I have just based my patch in the other functions.

Left TODO:
 * fix conjugate_cc
 * make sure the test fails if it is not implemented (I admit I didn't fully understood the testing procedure).