pdftotext isn't included on non-linux OSes

Extracting PDFs doesn't work on windows, because windows doesn't come with pdftotext:

```python
In [3]: textract.process("example.pdf")
---------------------------------------------------------------------------
ShellError                                Traceback (most recent call last)
<ipython-input-3-41fdbe49a77b> in <module>()
----> 1 textract.process("example.pdf")

c:\python27\lib\site-packages\textract\parsers\__init__.pyc in process(filename, **kwargs)
     24         raise exceptions.ExtensionNotSupported(ext)
     25
---> 26     return filetype_module.extract(filename, **kwargs)

c:\python27\lib\site-packages\textract\parsers\pdf.pyc in extract(filename, method, **kwargs)
      8     method = method or 'pdftotext'
      9     if method == 'pdftotext':
---> 10         return extract_pdftotext(filename)
     11     elif method == 'pdfminer':
     12         return extract_pdfminer(filename)

c:\python27\lib\site-packages\textract\parsers\pdf.pyc in extract_pdftotext(filename)
     17 def extract_pdftotext(filename):
     18     """Extract text from pdfs using the pdftotext command line utility."""
---> 19     pipe = run('pdftotext %(filename)s -' % locals())
     20     return pipe.stdout.read()
     21

c:\python27\lib\site-packages\textract\shell.pyc in run(command)
     16     # if pipe is busted, raise an error (unlike Fabric)
     17     if pipe.returncode != 0:
---> 18         raise exceptions.ShellError(pipe.returncode)
     19
     20     return pipe

ShellError: Command failed with exit code 1
```

```python
In [4]: import pdftotext
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-4-46fa7238b159> in <module>()
----> 1 import pdftotext

ImportError: No module named pdftotext
```

Maybe require pdftotext or xpdf support? http://en.wikipedia.org/wiki/Pdftotext