Fix core.system test

The find_device check in core.system is invalid. It could fail when same
device is supported by several platforms. In my case this happens for
Intel CPU when both AMD and Intel platforms are installed. The CPU
returned by boost::compute::system::default_device() is served by the
AMD platform, and the CPU returned by
boost::compute::system::find_device(name) is served by Intel SDK. The
only thing that could be safely asserted here is that both devices have
the same name.
Merge pull request #35 from ddemidov/fix-core-system-test

Fix core.system test
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.