Remove trailing whitespaces in messages

[yamgent]

Following up on https://github.com/nus-cs2103-AY1718S1/forum/issues/32. Since auto-stripping trailing spaces is a very useful feature (especially when we reach level4 and have to deal with checkstyle), perhaps a better solution would be to remove trailing whitespaces in messages on our side, so that developers don't have to disable the feature.

• Messages.MESSAGE_INVALID_COMMAND_FORMAT should have its trailing whitespace removed, to avoid the diff issue shown here.
• When printing list of tags, do not print a trailing whitespace when there are no tags (or just print "Tags: None"?), to avoid the diff issue shown here.
• Avoid printing a single whitespace character at the end of the list, to avoid the diff issue shown here.