Fixes from development

[stvkpln]

Lots of fixes!

• Starting to add in Pester unit testing capabilities (Thanks, Luc!!)
• Moving more global logging verbiage out of functions and into the root module as variables for ease of editing
• Making some internal Write-Warning part of Verbose logging, rather than having warnings coming up
• Renamed Confirm-FilePath to Confirm-FileExtension to better reflect what it actually validates
• Renamed switch parameter -NoClobber to -AllowClobber to better comply with switches not being 'true' by default
• Added more comments onto internal functions
• Added -Standard parameter on the Get-VirtualPortgroup check in the Confirm-BackingNetwork function
• Added all passthrough variables as parameters on Import-Appliance and fixing up code on appliance wrappers
• Added new function Get-FormattedMessage to format time and logging output to match up with PowerCLI date/time output
• Updated README

This PR should resolve issue #7