Add a command line param for test execution timeout.

Added a -timeout command line param with a 5 second default.  The command line value is only used if a *.goconvey
file does not contain a more specific value with "-timeout=[value]".

Users can send "-1s" for an infinite timeout (on the command line or in a *.goconvey file).
Merge pull request #260 from dlclark/master

Add a command line param for test execution timeout.