Reworked transform files to be xdt (un)install files instead

Less didn't need transforms as they are handled already by the dotLess dependency
Updated the version to 1.8.1 and made sure all of the nuspecs ask for that as the least version
The Build-Release.ps1 was requiring a PreReleaseName while it was not necessary, set to false

I know it's a bit more verbose but by specifically using config transforms you prevent nodes in the xml from jumping around during an update-pacakage. This helps people using source control to see what's actually changed. So for example if you updated now, the `<validation validateIntegratedModeConfiguration="false" />` node would jump from being the first child of `<system.webServer>` to being the last child of said node. Now it just updates the node if it exists (and keeps it in place) and adds it if it didn't.

I've tested all of the packages against a completely blank web.config to ensure everything gets created properly (In VS: File > New Project > Web > VS2012 > ASP.NET Empty Web App).

Further: See https://github.com/Shandem/ClientDependency/pull/41 for other updates (remove trailing spaces, erroneous version=1).

Lastly, (I didn't do this in this PR) but you should add a blank config\ClientDependency.config file when the file does not yet exist (so for anyone not using Umbraco) as currently the web.config will point to said file but if it doesn't exist then you just get an error running the site.