Documentation fixes

[mjsteinbaugh]

Improve website documentation and update the internal makeGRangesFromEnsembl() call to use build instead of genomeBuild.

[codecov-io]

Codecov Report

Merging #79 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #79   +/-   ##
=======================================
  Coverage   71.91%   71.91%           
=======================================
  Files          34       34           
  Lines        2357     2357           
=======================================
  Hits         1695     1695           
  Misses        662      662

Impacted Files	Coverage Δ
R/readCellRanger.R	82.84% <100%> (ø)	⬆️
R/bcbioSingleCell-constructor.R	77.13% <100%> (ø)	⬆️

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