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.
Merge pull request #15 from sergeykolosov/compress_level

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