Update coverage.rst
Merge pull request #62 from xubeisi/patch-1

Update coverage.rst
Update advanced-usage.rst
Merge pull request #64 from alexpenson/patch-1

Update advanced-usage.rst
add multiple file support for genomecov, only tested for :
genomeCoverageBed -ibams listbam -g chromsize
genomeCoverageBed -is listbed -g chromsize

options added:
    -is     The input list file is <bed/gff/vcf>.
    -ibams      The input list file each line is filename in BAM format.