[MERGE AFTER PREVIEW 4] Re-word include dialog instructions

Reword include dialog instructions as discussed in #114. It looks like this now: 

![words](http://f.cl.ly/items/0O1Q3l2c1t0V1e3i2O1C/Screen%20Shot%202013-05-21%20at%201.58.58%20PM.png "words")

This also collapses the two existing internationalized strings into a single string as a stop-gap until the new instructions are retranslated.

# DO NOT MERGE
... until adobe/brackets#3949 is committed.

CC: @ryanstewart @larz0