Fixed BorderTests.
Moved collections to their own namespace.
Make IReadOnlyPerspexList covariant.
Fixed a couple of typos

I assume they are typos anyway.. might improve the google-fu a small amount :)
Merge pull request #16 from grumpydev/patch-1

Fixed a couple of typos
Merge branch 'master' of https://github.com/SuperJMN/Perspex.git
Missed fix by Grokys!: TextBox collapses when no text: measure workaround!
Merge branch 'master'
Fixed exception when the text is null