Change default filename param to include :version

Without this commit, the default value used by the global configuration
does not properly handle version if it is not defined globally. There is
already functionality in `PEBuild::Archive` that will replace the
string ":version" with the version number passed from the merged
configurations.

This commit updates the default value for the global configuration
parameter "filename" to use the string ":version".
Merge pull request #16 from trlinkin/gsub

Change default filename param to include :version
Add comment about unfinalized values
Decipher some_mysterious_and_undocumented_variable
Only try to download installer if it's present
Remove README warning about executable dependencies

The dependencies have been removed.