Added a new class custom_stats for gathering statistical data.
Using the class you can conveniently use counters and timers
and activate/deactivate all of them at compile-time with a single
typedef line.
Added three new macros to verbose.h
    - STXXL_VARDUMP(x) prints the name of x together with its value.
    - STXXL_MEMDUMP(x) prints the name of x together with its value as an amount of memory in IEC units.
    - STXXL_CHECK_EQUAL(a,b) is an assertion macro for unit tests, similar to
        STXXL_CHECK(a==b). The difference is that STXXL_CHECK_EQUAL(a,b) also prints
        the values of a and b. Attention: a and b must be printable with std::cout!
* Added a new bulk-parallel priority queue class for external memory: parallel_priority_queue.h
    The algorithm is described in detail in http://algo2.iti.kit.edu/2386.php

* Added a new benchmark benchmark_pqs.cpp for comparing different priority queue implementations on the same input data.