repl: backslash bug fix

[thefourtheye]

1st commit:

When some string is copied and pasted in REPL, if it has a newline (\n)
in it, then it is considered as end of line and REPL throws a
SyntaxError, because a line cannot end with a unclosed string literal.
This behavior is because of the fact that readline module breaks at
all the \ns in the input string and treats them as a seperate line.
So whenever it encounters a new line character it will emit a line
event.

This commit basically reverts 81ea52a, which was an attempt to improve
the way string literals were handled by REPL.

Fixes: #2749

2nd commit

As it is, the trimWhitespace function will not remove the white sapce
characters at the end of the string, as the greedy capturing group .+
captures everything till end of the string, leaving \s* with nothing.

This patch replaces the buggy function with the built-in, String
prototype's trim function.

Note: The second commit is necessary because, the revert brings back the old buggy trimming function. So, I though we could fix it now itself.

cc @silverwind @Fishrock123

[silverwind]

LGTM, but your description regarding \n seems inaccurate, I see it happening with any backslash escape sequence:

> function x() {
...   return '\\';
SyntaxError: Unexpected end of input

[thefourtheye]

```
➜  io.js git:(master) node      
> process.version
'v2.5.0'
> function x() {
... return '\\';
SyntaxError: Unexpected end of input
    at Object.exports.createScript (vm.js:24:10)
```

---

Edit: Sorry, just realized that this change landed in 2.5.0 only. It used to work fine in 2.4.0. Trying to find the actual problem now.

[thefourtheye]

@silverwind I could not be more wrong. This problem has nothing to do with the readline module at all. Closing this, as I have opened a new PR with the proper fix. PTAL. Sorry for all the fuss.