IsNull and DropNA expressions

closes #697 
@mrocklin 

pandas: `<column>|<table>.dropna()`
sql: `select * from <table> where <column> NOT NULL`
mongo: `db.<collection>.find( { '$and': [ { 'cancelDate': None }, { 'cancelDate': { '$type': 10 } } ] )`
scidb: `SELECT expression1 FROM array WHERE not is_null(expression2);`, see here: http://scidb.org/HTMLmanual/14.3/scidb_ug/re11.html
python: `filter(None, [...])`
numpy: `x[~np.isnan(x)]` if you're using regular arrays, `x.compressed()` if you're using `MaskedArrays`