[SPARK-8218][SQL] Add binary log math function

JIRA: https://issues.apache.org/jira/browse/SPARK-8218

Because there is already `log` unary function defined, the binary log function is called `logarithm` for now.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #6725 from viirya/expr_binary_log and squashes the following commits:

bf96bd9 [Liang-Chi Hsieh] Compare log result in string.
102070d [Liang-Chi Hsieh] Round log result to better comparing in python test.
fd01863 [Liang-Chi Hsieh] For comments.
beed631 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_binary_log
6089d11 [Liang-Chi Hsieh] Remove unnecessary override.
8cf37b7 [Liang-Chi Hsieh] For comments.
bc89597 [Liang-Chi Hsieh] For comments.
db7dc38 [Liang-Chi Hsieh] Use ctor instead of companion object.
0634ef7 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_binary_log
1750034 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_binary_log
3d75bfc [Liang-Chi Hsieh] Fix scala style.
5b39c02 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_binary_log
23c54a3 [Liang-Chi Hsieh] Fix scala style.
ebc9929 [Liang-Chi Hsieh] Let Logarithm accept one parameter too.
605574d [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_binary_log
21c3bfd [Liang-Chi Hsieh] Fix scala style.
c6c187f [Liang-Chi Hsieh] For comments.
c795342 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_binary_log
f373bac [Liang-Chi Hsieh] Add binary log expression.