BUG: pd.eval handles operator not as operator ~
```
>>> import pandas as pd
>>> pd.eval('not True')
-2
```

Not sure if this was intentional, since `'not'` maps to `op.invert` in https://github.com/pydata/pandas/blob/master/pandas/computation/ops.py#L458. After fixing that `numexpr` still `invert`s rather than `not`s the operand.

```
INSTALLED VERSIONS
------------------
commit: 53a0db18104abde098d43c66d77555fad12ed8c1
python: 3.4.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-63-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.17.0+62.g53a0db1
nose: 1.3.6
pip: 7.1.2
setuptools: 18.4
Cython: 0.22
numpy: 1.9.2
scipy: 0.15.1
statsmodels: None
IPython: 4.0.0
sphinx: 1.3.1
patsy: 0.3.0
dateutil: 2.4.2
pytz: 2015.4
blosc: None
bottleneck: 0.8.0
tables: 3.1.1
numexpr: 2.4.3
matplotlib: 1.4.3
openpyxl: 2.0.3
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.7.6
lxml: 3.4.4
bs4: 4.3.2
html5lib: 1.0b2
httplib2: None
apiclient: None
sqlalchemy: 1.0.4
pymysql: 0.6.6.None
psycopg2: None
```
