standardise on double quotes in readme

* single quotes don't render in Chrome when markdown is translated on CRAN
Add nf.conv argument to clampmax to replace non-finite values
Merge branch 'clampmax' of github.com:ajdm/nat into ajdm-clampmax

* 'clampmax' of github.com:ajdm/nat:
  Add nf.conv argument to clampmax to replace non-finite values
  Update .Rbuildignore for Travis changes
full set of tests for clampmax
nf.conv -> replace.infinite and replace Inf values with NA before max

* note that this is the intended behaviour since clampmax(1,2,Inf) should
  return 2 not Inf
Merge branch 'ajdm-clampmax'

* ajdm-clampmax:
  nf.conv -> replace.infinite and replace Inf values with NA before max
  full set of tests for clampmax
  Add nf.conv argument to clampmax to replace non-finite values
  Update .Rbuildignore for Travis changes
Merge branch 'master' of github.com:jefferis/nat

* 'master' of github.com:jefferis/nat:
  Correct expected values for plot.neuron test
  Simplify axis handling in plot.neuron
  Add clampmax example to projection()