Use Get-EnvironmentVariable method

Changed how the environment variable for the chocolatey install
directory was retrieved. Using the $env: syntax won't find an
environment variable that was just added, it only pulls variables that
existed when the process was created. Calling the Get-EnvironmentVarible
method queries the registry directly.

This lets you install and use chocolatey without having to restart the
Octopus tentacle or the machine after the installation. This is also a
handy way to check environment variables that may have been created
after your process was spawned.
Fixed Typo, Added compression support

I found and fixed a typo when assigning the SqlPassword parameter to the
SqlPassword powershell variable. There was an extra $ which caused the
SQL backup step to fail.

I also added support for a CompressionOption parameter so that the
database backups can be compressed to save space.
Merge pull request #55 from joewaid/master

Updates to Chocolatey Ensure Installed, Chocolatey Install Package, SQL - Backup Database