refactor(launcher): skip the child process if only one capability is requested

Closes #603
fix(locators): Improve custom locators message

Increase readability of custom locator message by displaying each argument instead of the arguments object.
chore(tests): fix up unnecessary 'more than one element' warnings
Closes #661
docs(jasmindwd): throw an error if tests attempt to use a 'done' callback
Closes #659
chore(jasminewd): fix the prior fix for documentation failures with done
fix(pageload): Changing how `about:blank` unload waits
Also changing `executeScript` script comment from `//` to `/**/`
format. These two small changes should not affect functionality but
make Protractor work with Selendroid.
chore(clientsidescripts): update all scripts to avoid // comments
chore(release): update version and changelog for 0.21.0
chore(release): update API documentation for 0.21.0
chore(git): add npm debug log to gitignore
chore(webdriver): update selenium standalone to 2.41
fix(webdriver-manager): fix download paths
fix(webdriver-manager): do not download files if HTTP response is not 200

Closes #656
WIP : Added first draft