Fixed nth_element, added unary_nor

[koosha94]

This commit adds unary_nor, and nor1 as a logical operator.
It also adds/fixes support for user defined types in nth_element.

[jszuppe]

Thanks for that! It's a good idea, but I have some comments:

• NOR is a binary operator (negation of OR).
• NOR operator can be constructed just using binary_negate and logial_or:

not2(logical_or(arg1, arg2))

• Don't use master branch for pull requests, make a special feature branch (in this case it could be called fix-nth-element' or whatever), because now yourmaster` and Boost.Compute are different and that can make problems when you want to make next pull request.
• It would be great to have test for user-defined types for nth_element.

Maybe it would good to add find algorithm version that works with user defined types (Compare or Equal binary function would be required)?