JSXTransformer, concatenate consecutive strings

JSXTransformer currently produces suboptimal code when fixed strings are inserted with braces ```{'Abc'}```, sending them as separate arguments to React.DOM rather than concatenating them and sending them as one string. Each argument is turned into a separate string, so this actually reduces the number of spans generated for plain text.

There are a few special-cases where my PR does not concatenate, but they are pretty odd/rare and probably not worth the extra complexity to catch.

I also removed a duplicated piece of code.

### Example
```
<div>
  This {'the'} {}{x+x}{' '}{'story'}.
  That story.
  
  Of the snow{' '}
  white space.
</div>
```

### Before
```
React.DOM.div(null, 
  " This ", 'the', x+x,' ','story',". "+
  "That story. "+
  "Of the snow",' ',
  " white space. "
)
```

### After
```
React.DOM.div(null, 
  " This "+ 'the', x+x,' '+'story'+". "+
  "That story. "+
  "Of the snow"+' '+
  " white space. "
)
```