Check write permissions before writing .nrepl-port

Fixes #1664
Clarify .lein-classpath usage with plugin deps

Explain that plugin dependencies need to be added to .lein-classpath if they are being edited too.
with->on
Correct classpath separator config
Merge pull request #1767 from danielcompton/patch-1

Clarify .lein-classpath usage with plugin deps
Merge pull request #1766 from ernestas/check-for-write-permissions

Check write permissions before writing .nrepl-port
Merge remote-tracking branch 'upstream/master'