Add ability to send log output to a file

Using PACKER_LOG=1 causes all the log output to be sent to Stderr. This
change maintains that backward compatility. Anything other than "1" will
be treated as a filename which will have logging appended to that file.
This is useful, for example, to always have debugging available without
cluttering up stdout (and without having to redirect stderr all the time).
Add PACKER_LOG_PATH for logging to a file

When the environment variables PACKER_LOG and PACKER_LOG_PATH are both
set the log output will be appended to the PACKER_LOG_PATH file.
Merge pull request #168 from markpeek/markpeek-filelogging

Add ability to send log output to a file using PACKER_LOG_PATH