test basic string conversion, expose bug

It does not work as advertised:

In [1]: import pypandoc

In [2]: output = pypandoc.convert('#some title', 'rst', format='md')
---------------------------------------------------------------------------
IOError                                   Traceback (most recent call last)
<ipython-input-2-031b23b69b18> in <module>()
----> 1 output = pypandoc.convert('#some title', 'rst', format='md')

/home/esc/anaconda/lib/python2.7/site-packages/pypandoc/pypandoc.pyc in convert(source, to, format, extra_args)
      8     Raises OSError if pandoc is not found! Make sure it has been installed and is available at path.
      9     '''
---> 10     return _convert(_read_file, _process_file, source, to, format, extra_args)
     11
     12 def _convert(reader, processor, source, to, format=None, extra_args=()):

/home/esc/anaconda/lib/python2.7/site-packages/pypandoc/pypandoc.pyc in _convert(reader, processor, source, to, format, extra_args)
     11
     12 def _convert(reader, processor, source, to, format=None, extra_args=()):
---> 13     source, format = reader(source, format)
     14
     15     formats = {

/home/esc/anaconda/lib/python2.7/site-packages/pypandoc/pypandoc.pyc in _read_file(source, format)
     41
     42 def _read_file(source, format):
---> 43     with open(source) as f:
     44         format = format or os.path.splitext(source)[1].strip('.')
     45         source = f.read()

IOError: [Errno 2] No such file or directory: '#some title'
test basic file conversion
fix string conversion, all tests now pass
Merge pull request #4 from esc/fix_converting_strings

fix for #3 - using test driven development to expose and fix bug