renamed SlaveOption class to CreateSlaveOption

CreateSlaveOption is a more descriptive name, and follows the
naming style of other subcommand's options classes.
add local assertRaisesRegexp() on python older then 2.7

When running tests on python older then 2.7, monkeypatch
unittest.TestCase class to include assertRaisesRegexp() method.
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