Re-use existing connections

Check for existing connections to the vCenter server and re-use if they
exist. Saves duplicate connections and results.
Separate HTML template from code

Separated all HTML code from the vCheck.ps1 script to allow creation of
Style templates, migrated existing HTML code into a Default style.
Delete Header.jpg

Moved to /Styles/Default/Header.jpg
Added $Style setting to GlobalVariables
Changed to use Style folder structure
Add $Style=Default to GlobalVariables
Fixed style selection

Fixed style selection bug

Added "CleanGreen" example style
Merge pull request #49 from sammcgeown/master

Split HTML and CSS to a "Style" file