Options should reflect xpdf options, not spawning options

Currently the options object does not give any control of how xpdf should parse the PDF. Instead it gives you options for how to control the spawning process which seems to be an implementation detail not relevant for the client.

I suggest using the options object to control xpdf. I decided to go with an array instead of an object for the options argument since it was the quickest fix and does not require constant updates to this module should xpdf decide to change the list of arguments. It does break backwards compatibility though.
Fixed options argument being changed
Added arguments to options object

I decided to refactor things a little to make it backwards-compatible. The options parameter is back and the arguments to xpdf can be configured by using the options.args object.
Merge pull request #6 from peterhaldbaek/options

Options should reflect xpdf options, not spawning options