#6430 unicode names

This merge simply replaces hex encoded unicode characters with named unicode characters for clarity.  It does not change any logic or functionality.  Names were obtained by checking them against unicodedata.name() so all replacements are functionally equivalent.  Any notations in comments were not changed.  Changes also limited to python files and tests within sympy/.  

Example:

```python
>>> import unicodedata
>>> print unicodedata.name(u'\u03c0')
GREEK SMALL LETTER PI
>>> u'\N{GREEK SMALL LETTER PI}'
u'\u03c0'
>>> u'\N{GREEK SMALL LETTER PI}' == u'\u03c0'
True
>>> u'\N{GREEK SMALL LETTER PI}' == u'\u03c1'
False
```