fix files as live object, make parser context new for each less file

Hi,

I was trying to compile twitter bootstrap with component less and found 2 issues:
1) ```var files = builder.config.styles;``` files here is a 'live' object and when we do 				
```
builder.addFile('styles', newFile, css);
builder.removeFile('styles', file);
```
we actually edit it, which breaks stuff

2) less is compiling one file at a time. And if we use same parser instance it breaks imports for all files after first one.

Here are my fixes