Fix codecs search function to only match 'symbol'

[somakrdas]

The current search function matches all encodings, which break other packages that use the codecs registry like cssutils. According to the codecs documentation:

In case a search function cannot find a given encoding, it should return None.

This pull request fixes the search function to only match 'symbol'.

Before:

>>> from pyth.plugins.rtf15.reader import Rtf15Reader
>>> import codecs
>>> print codecs.getdecoder('symbol')
<function symbol_decode at 0x109774140>
>>> print codecs.getdecoder('<some_encoding>')
<function symbol_decode at 0x109774140>

After:

>>> from pyth.plugins.rtf15.reader import Rtf15Reader
>>> import codecs
>>> print codecs.getdecoder('symbol')
<function symbol_decode at 0x1043b3140>
>>> print codecs.getdecoder('<some_encoding>')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/somakrdas/.envs/work/lib/python2.7/codecs.py", line 946, in getdecoder
    return lookup(encoding).decode
LookupError: unknown encoding: <some_encoding>

[somakrdas]

@brendonh, could you take a look and, when possible, release to PyPI? (pyth still conflicts with other packages that use codecs for us.)

[brendonh]

Seems reasonable!

[somakrdas]

Great, thanks!