closes #125 - search error when using : prefix

I accidentally stumbled across this the other day and now i've figured out how to set the OrchardDocs site up on my computer I thought I'd take a look at it.

It seems starting a search with a colon triggering an advance query parser but its not a valid one. Would have expected it to just fail silently but it doesnt. Instead you see this error:

    Encountered " ":" ": "" at line 1, column 0.
    Was expecting one of:
       <NOT> ...
       "+" ...
       "-" ...
       "(" ...
       "*" ...
       <QUOTED> ...
       <TERM> ...
       <PREFIXTERM> ...
       <WILDTERM> ...
       "[" ...
       "{" ...
       <NUMBER> ...

I did some searches and the standard approach is:

  1. Attempt an advanced query
  2. Catch the exception
  3. Pass it through a query sanitizer and try again

I've used the [code in this answer](http://stackoverflow.com/a/855189/156388) and it now accepts both the "broken" query in the issue and a normal text query.