bcftools index now works on vcf and bcf; defaults to .csi

* add vcfindex.c to do indexing of vcf and bcf; default at the moment to .csi

* move bcf_idx.c to bcf_idx.unused.c.

* hide tabix command. if we're going to remove it, we need to rejigger the tests that need to index tab delimited files.

* add `index` to the man
tests for bcftools indexing

* test limits of index retrival

* tbi limit is (`1<<29`). no error reported when creating the index though

* csi index is (`1<<31-1`). bcf creation and indexing a problem above this

* added commented out test for region specified as `chr20` rather than `chr20:1-2147483647`; fails at the moment, but shouldn't
Merge pull request #25 from sm15/feature/bcftools_index

bcftools index to work on VCF and BCF