Allow modifying the socket chunk-size

[exhuma]

Currently the chunk-size for socket reading is hardcoded to 4096 bytes. This value could be made configurable for the end-user. This is kind of related to #45 as it may reduce the chance of running into #45. However, fixing #45 has priority!

This is also related to #1 as it is another configuration value for the network behaviour.

Currently the timeout value is exposed by most top-level functions. To guarantee backwards-compatibility a new argument chunk_size should be introduced.

For the next major release, this would make more sense in the creation of an SNMP-Client instance (see also #3)

[exhuma]

The value is currently accessible via the global module-level variable puresnmp.transport.BUFFER_SIZE.

It can be modified by setting the value before making a call to an SNMP function.