Nasty port of dirtybench to gdbm (issue #64)

Tested on Linux 3.12 on Core i5-2300

GDBM:
    [insert] done all 787232 in 2.38s (330089/sec)
    enum 787232 (key, value) pairs took 0.89 sec
    rand lookup all keys 1.40 sec (561796/sec)
    per txn rand lookup+hash all keys 1.43 sec (550106/sec)
    get+put all 787232 in 2.77s (284007/sec)
    replace all 787232 in 1.45s (544343/sec)

py-lmdb master:
    [insert] done all 787232 in 1.36s (578565/sec)
    [insert writemap] done all 787232 in 0.45s (1759417/sec)
    [insert writemap, reused cursor] done all 787232 in 0.41s (1933746/sec)
    enum 787232 (key, value) pairs took 0.11 sec
    rand lookup all keys 0.91 sec (869255/sec)
    per txn rand lookup+hash all keys 1.20 sec (653872/sec)
    get+put all 787232 in 1.07s (736999/sec)
    replace all 787232 in 0.83s (954006/sec)