consistency and cosmetic updates to help messages

- spacing consistency across help messages
- try to make the usage line conistent across modules
- fix some typos
consistency with samtools for version and usage reporting

- bcftools version/--version/--version-only as in samtools
- "Version" line in main usage message consitent with samtools
- bcftools help/--help (to stdout and return 0)
usage displays for .vcf.gz only

- include note on main usage page about indexed/un-indexed VCF/BCF and streams
Merge pull request #28 from sm15/feature/cosmetics

consistency and cosmetic updates to help messages