Use new way of setting environment variables

Just an update to stop using a deprecated way of setting environment variables. No functional difference.

Optional prefixes to use multiple build numbers in one repository

This release adds an optional prefixparameter which will be pre-pended to the build-number-n tag, allowing you to have multiple build numbers in one repository, for example for a client and a server. See Readme.md for more detailed instructions. Thanks to @Brymastr for the contribution.

Generate build number - first release.

Initial release, see README for details on how it works.