Describe BAI changes for 'samtools idxstats' and details of BIN value

(Original commit 16 Mar 2012, rebased 8 April 2014)
Add n_no_coor to index formats given that it has been being written by samtools for a few years.
Correct final BAI bin number

There is an even number of bins on the last row too -- previously this
was typoed or off-by-one.  Hat tip Pavel Sinitcyn.
Move "optional" to the first column

Also regularise \cline and fix tabix header typo.
Describe pseudo-bins in a separate table

Hopefully makes clearer that the pseudo-bin is just another bin -- in
fact it doesn't even need to be the final bin in the list!  Add footnote
explaining placed/unplaced.
Merge BAI/CSI/tabix idxstats details (PR #2)
Remove SAM's artificial 'i' optional field range limit

SAM is a textual format, so there is no intrinsic limit on the number
of digits in an 'i' value.  In particular, SAM should be able to express
at least the same range as BAM, namely -2^31 (as 'i') to 2^32-1 (as 'I').

Fixes #36.