Automatically switch to an unused port

[adamralph]

Currently the default port is hardcoded to 8888.

If another process is using this (e.g. Fiddler defaults to 8888), then the following error message is given and the script exits:

Cannot access a disposed object. Object name: 'System.Net.HttpListener'.

If 8888 is busy, we should failover to 8889 and so on...

[adamralph]

Also relates to https://github.com/adamralph/scriptcs-nancy/issues/7 and https://github.com/NancyFx/Nancy/issues/1086