Use array.array('c') as a type of incomming buffer

A connection incoming buffer is extended using += operator. This leads
to unnecessary memory allocations and significant performance drops when
transferring big amounts of data (i. e. a few megabytes). Using of the array('c')
(an array of bytes) data type enables us to handle incoming data efficiently.
Merge pull request #33 from malor/master

Fix slow handling of connection incoming buffer