Pickling of Frames

[st-pasha]

• Frame now supports pickle interface;
• when copying a Frame, its key property is no longer lost;
• Frame.save() can now also be called without any arguments -- it will save the Frame into memory in Jay format, and return the bytes object containing the Frame's data;
• dt.open() can now accept a bytes argument -- same as returned from Frame.save().

Closes #1442
Closes #1443