Indicate error with exit code

This way, errors can be caught when using glacier-cli from a shell script
or command line.
Merge pull request #21 from winny-/exitCode

Indicate error with exit code
Write usage and errors to System.err

This follows the behavior of uncaught exceptions, which are printed to
standard error. See:
https://stackoverflow.com/questions/11924348/where-are-unhandled-exceptions-in-command-line-java-application-printed-out-by-d

This commit additionally fixs an usage formatting error where there
are missing spaces in the list of possible glacier-cli commands.
Add -help flag & print usage on invalid flag

Also removed the check for the properties file, as PropertiesCredentials does
this for us and raises the appropriate exception.
Add -properties argument to specify the path of AwsCredentials.properties.

I thought about adding -secretkey and -accesskey arguments, however stealing
access data via /proc is rather easy, so why encourage it?

Also updated README.md's usage text.
Merge pull request #22 from winny-/moreCLIImprovements

More cli improvements