use the connection buffer for writing
writeExecutePacket: fix capacity check
writeExecutePacket: fix packing
various refactoring
more refactoring

Try to remove unnecessary indirections and initialisations with zero.
Also update links to the MySQL doc
buffer: rename take buffer funcs
changelog: Add write buffer
writeCommandPacketUint32: fix packet length comment
packets: YAR (yet another refactoring)
Merge pull request #134 from go-sql-driver/write-buffer

Use the Connection Buffer for Writing