Add set_continue() function to __init__.py

Add info about linecache to the NullSourceCodeProvider message

Fixes #238.

paused=True is the default. This also automatically causes the set_trace to
not be treated as a breakpoint, since it doesn't break there.

Add a docstring to set_trace() and updated the docstring of
Debugger.set_trace().

Fix NullSourceCodeProvider rendering

Don't use SetPropagatingDict in the built in console

Passing a dict subclass to eval is not supported in Python 2. This was causing
weird issues where the C-x would break the UI (I am not clear exactly how it
caused those issues).

The point of SetPropagatingDict is to combine globals and locals into a single
dictionary for things like code.InteractiveConsole and rlcompleter.Completer,
which take a single dictionary, but to allow changes to propagate, so that
assignments in the shell can modify global names. However, this is completely
unnecessary for the built in shell, as we call eval() ourselves, so we can
just pass the exact globals and locals dicts separately.

This fixes #166.

This lets you start the debugger and immediately continue, i.e., it only stops
on a previously defined breakpoint.

Fixes #234.

set dont_inherit to ignore future statements when compile file

fix bug compile failed in Python 2.X

Make Debugger.interaction match the signature of Pdb.interaction.

This allows pudb to be used as debugger for pytest>=3.0, e.g.

```
$ cat test_foo.py
def test_1():
    assert False
$ pytest --pdbcls pudb.debugger:Debugger --pdb --capture=no
```
(starts pudb at the failed assertion site).

Note the need to pass `--capture=no` (or its synonym `-s`) as otherwise
pytest tries to manage the standard streams itself.

Backwards compatibility is preserved.

Fix ptpython embed

Setting term_size to the current size if None