Added log10 to the list of unary functions df.eval can handle
- [x] closes https://github.com/pandas-dev/pandas/issues/24139 
- [x] tests ~~added~~ / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

`(pandas) ➜  pandas git:(binary_math_log) git diff upstream/master -u -- "*.py" | flake8 --diff
`
- [ ] whatsnew entry

