refactor(distro): use default linux browser if configured

- use environment variable BROWSER if already set
- use gnome or X default browser if configured
- refactor browser executable search to be more extensible
- use echo -e option to enable interpretation of backslash escapes
  (most distributions disable this by default)
- remove redundant spaces
merge(start-script): add better browser handling

merges Pull Request #53

Conflicts:
	distro/gf31/assembly/src/start-camunda.sh
	distro/jbossas7/assembly/src/start-camunda.sh
	distro/tomcat/assembly/src/start-camunda.sh