Improve the monte carlo example

This improves the monte carlo example by using the count_if()
algorithm instead of a custom kernel with atomics. Also includes
only the required headers instead of all the Boost.Compute headers.
Rename find_longest_vector example to longest_vector
Add price_cross example

This adds a new example demonstating the use of find_if() and
zip_iterator to find the point at which two vectors of price values
cross each other.