Fix: Is now working w/o username and password too
Only source ./config.cfg if it is readable

Since defaults are already specified ./config.cfg does only need to be
sourced when it is readable (and therefore exists). This prevents an
error and the script abortion if the config file is not present, what
makes no sense since the defaults can be used.
Fix: Using /usr/bin/env in shebang to locate sh

Since sh has not to be in /bin/sh in any case. Using /usr/bin/env sh
instead makes the script more environment independent.
Merge pull request #1 from olsaki/master

Some fixes to make mongo-migrate a little bit more flexible