renamed SlaveOption class to CreateSlaveOption

CreateSlaveOption is a more descriptive name, and follows the
naming style of other subcommand's options classes.
catch invalid numeric parameters to 'buildslave create-slave' command

When converting 'usepty', 'keepalive', 'maxdelay' and 'log-size' parameters
to ints, catch ValueError exceptions and print error message. This way it is
easier to troubleshoot incorrect parameters to create-slave command.
accept exactly 4 arguments to 'buildslave create-slave' command

Check that 'buildslave create-slave' command was invoked with exactly 4
arguments. Previously when create-slave was given more then 4 arguments, an
'ValueError: too many values to unpack' exception was raised.
added unit tests on CreateSlaveOptions class

Test arguments parsing of valid 'buildslave create-slave' commands.
test '<master> not URL' error for 'buildslave create-slave' command