Remove MIT License, move License.txt to root, update build script

[SteveL-MSFT]

Microsoft Reviewers: Open in CodeFlow

[daxian-dbw]

@SteveL-MSFT License.txt still has Copyright (c) 2013, Jason Shirk as the first line. Should that be changed to Copyright (c) Microsoft Corporation?

[iSazonov]

Isn't the project now under MIT license?

[daxian-dbw]

@iSazonov After consulting with the corporate attorney, it's been decided to keep the original BSD license for PSReadLine.

[iSazonov]

@daxian-dbw Thanks! Should we add this ("BSD license") in the file, readme and GitHub repo description explicitly?
Also the PR description could be "Replace MIT license with BSD..."

[daxian-dbw]

I need to confirm with @joeyaiello on whether I should refresh all files with a new header that calls out the BSD license. As for the repo itself, the readme.md already calls out the license, at the end.

[iSazonov]

Also MSFT could want to preserve Jason's copyright and add MSFT copyright.

[daxian-dbw]

Updated the PR to resolve conflicts.
I will go merging this PR. If we need to update the license.txt I will submit a new PR.