Allow the default wait timeout to be configurable

Allows configuration of the default wait timeout used by the wait methods - wait_until, wait_while, when_present, wait_while_present and wait_until_present:

````ruby
Watir.default_timeout = 60
````

This is similar to a recent change in Watir-Webdriver (watir/watir-webdriver#230). As well, this changes the default timeout to be 30 seconds for consistency.