fix handling in jlkernel execute_request

[stevengj]

We have a barely functional execute_request, with many omissions:

• Currently, we don't really handle exceptions well at all. Related to pyerr messages in Support all message types in jlkernel #4.
• We don't capture stdout/stderr (see Julia #3823)
• Results are printed via repr, but we should instead print just like they are printed in the REPL

[stevengj]

Also, we are currently ignoring everything in content except for code. We also need to take into account the other fields:

• silent - if true, silence stderr, no history, not sure what else
• store_history
• user_variables and user_expressions - need to evaluate and return in execute_reply
• allow_stdin (depends on redirect stdin to frontends #42) - if false, throw exception if read from stdin? or treat stdin as empty file? @fperez?

@fperez, what do we do if evaluating user_variables or user_expressions throws an exception? Also, can we assume that user_expressions are Julia expressions or do we have treat it as Python code?

[stevengj]

Also, we should:

• suppress output (and history caching) if the code ends with ;, similar to the current Julia REPL.

[stevengj]

Closing this since it is mainly just a matter of implementing #42 at this point.