Series/DataFrame element-wise equality operations

[andrus]

Let's define a few element-wise logical operations for DataFrames and Series, all returning a DataFrame / Series of booleans (that can be later used as value replacement conditions per #31) :

BooleanSeries mask = s1.eq(s2);
BooleanSeries mask = s1.ne(s2);

DataFrame mask = df1.eq(df2);
DataFrame mask = df1.ne(df2);

DataFrames being compared must have the same number of rows and columns and the same column labels.