Fix issues involving iterators with void value_type

This fixes a few issues encountered when using iterators with a
void value_type (e.g. std::insert_iterator<>).

The is_contiguous_iterator meta-function was refactored to always
return false for iterators with a void value_type and avoid
instantiating types for containers with a void value_type
(e.g. std::vector<void>::iterator) which previously resulted
in compilation errors.
Add support for get<N>() in lambda expressions

This adds support for using the get<N>() function in lambda
expressions to extract a single component of an aggregate type.

Also adds a test of using boost::tuple<> to store a user-defined
data type on the device and sort them by their first component
using a lambda expression as the comparator.
Add inplace_merge() algorithm

This adds a simple inplace_merge() algorithm which merges
two contiguous sorted ranges in-place.

For now, the implementation simply copies the ranges to
two temporary vectors and calls merge().
Cleanup example code

This cleans up the example code. Now all of the examples use
the "namespace compute = boost::compute" alias. This shortens
the example code making it less verbose and more clear. Also
cleans up a few style issues.
Remove timer class

This removes the timer class. The technique of measuring the time
difference between two different OpenCL markers on a command queue
is not portable to all OpenCL implementations (only works on NVIDIA).

A new internal timer class has been added which uses boost::chrono
(or std::chrono if BOOST_COMPUTE_TIMER_USE_STD_CHRONO is defined).
This new timer is used by the benchmarks to measure time elapsed
on the host.
Cleanup documentation

This cleans up and improves the documentation.