Syntax error fix

- "mkdir -p" is a linux command, not windows ;)
- Slashes wrong direction, they trigger a syntax error.
- Quotation marks wrong, they trigger a syntax error.
Folder allready exists

Folder already exists, can be removed.
;!system "mkdir ..\..\build\releases\Popcorn-Time\win\"
Update installer.nsi

No I was wrong... it doesn't exists yet.
Merge pull request #856 from r0gger/patch-1

Fixing slash direction in the installer NSI