Introduced a setting to configure 'compress_level' (for pylibmc 1.3.0+).

Allows to set the compression level via the Django setting
`PYLIBMC_COMPRESS_LEVEL`. The setting value is respected only for
pylibmc 1.3.0 and above, otherwise defaults to `1` (`Z_BEST_SPEED`)
which is hardcoded into the pylibmc itself.

This commit fixes the `test_too_big_value` test failure for pylibmc 1.3.0.

Also, I took the liberty of setting the minimum required version of pylibmc inside the  `install_requires` of `setup.py` (it is in fact the minimum version which is compatible with django-pylibmc).

It clashes a bit with #14 inside `setup.py`, so in case either of these is merged to master, I'll rebase the other one for the second merge to go smoothly.