commands: add append and prepend commands

Added a new error code OMCACHE_NOT_STORED which maps to memcached's
PROTOCOL_BINARY_RESPONSE_NOT_STORED which is issued for appends and prepends
to non-existent keys.  Also added omcache.py mapping for TooLargeValueError
and DeltaBadValueError which were introduced earlier on but lacked mappings
in the Python bindings.

As version 0.1.0 was already tagged add the new symbols to OMCACHE_0.2 which
will be the next release.
commands: add touch command
python delta operations: don't initialize value by default

Delta operation definition in the protocol is a bit weird; if 'expiration'
is set to DELTA_NO_ADD (0xffffffff) the value will not be created if it
doesn't exist yet, but since we have a 'initial' argument in the python api
we'd really like to use it to signal whether or not a value should be
initialized.

To do that we'll map expiration to DELTA_NO_ADD if initial is None and
expiration is empty and throw an error if initial is None but expiration
isn't empty.
omcache_libmemcached.h: add append, prepend, touch, increment, decrement

Append, prepend and touch are new commands since 0.1.0, increment and
decrement were already available previously, but only as the _with_initial
variants.

Also moved some definitions around a bit and renamed macro arguments to
avoid conflicts with 'exp'.
logging: fix null pointer dereference when logging is not enabled

Missing braces around log priority in the if clause caused the log call in
omc_srv_disable() which uses a higher priority to log messages when first
disabling a server caused the "should we log" check to always result in true
as the macro expanded the log call:

    #define omc_log(pri,fmt,...) ({ \
        if (mc->log_cb && pri <= mc->log_level) { \
            ...
    omc_srv_log(srv->disabled ? LOG_INFO : LOG_NOTICE, ...

into:

    if (mc->log_cb && srv->disabled ? 6 : 5 <= mc->log_level)

Reported by Piotr Szymański.