Fold enhancements

I've added some improvements to the folding.

For the simple folding method (folding all pairs of { }) I just added ( ) and [ ] as folded pairs so arrays can be folded.

The advanced folding (php_folding==1) got a lot more improvements:
* try/catch folding was improved to allow for individual folding for the try and catch(es) blocks, even if the catch is on the same line as the closing } for previous block.  Folding for finally blocks was added.
* for, foreach, while, and do/while loops are all now folded.
* if/elseif/else blocks are now folded, with each block being folded individually much like try/catch/finally.
* switch blocks are now folded.  First, the switch block as a whole folds, then each case block and the default block fold within that.
* heredocs and nowdocs now fold.

There are a couple of bugs I couldn't figure out how to fix:
1. Closing } highlighting.  I could not figure out how to get the closing } in try/catch/finally and if/elseif/else to highlight as a delimiter when they were followed by the next block's keyword on the same line, e.g. 
```php
    if ($expr) {
        // true
    } else {
        // false
    }
```
2. I couldn't figure out how to get successive case fall-throughs to fold together, only the last case statement will be in the fold, leaving the previous ones unfolded. e.g. 
```php
    switch ($expr) {
        case 0: // fall-through, not folded.
        case 1: // fall-through, not folded.
        case 2: // do something, folded.
            // code
            break;
    }
```
Is the only difference in the `else` regions the `fold` keyword at the end of each line? If this is the case, we can do something similar to the [built-in .yacc](http://ftp.vim.org/pub/ftp/ftp/vim/runtime/syntax/yacc.vim) syntax:

```vim
" ---------------------------------------------------------------------
"  Folding Support {{{1
if has("folding")
 com! -nargs=+ HiFold	<args> fold
else
 com! -nargs=+ HiFold	<args>
endif

" Usage:
HiFold syn	region ...
```