implemented the @pause gcode host command (and a small framework for host commands)

Pause code (Ported from RepetierHost, untested)

Imported and translated from RepetierHost

First commit unfinished implementation

experimental branch: fixing broken pyreadline download url

fixing broken pyreadline download url

[printcore] Have print_thread rejoin execution thread.

[printcore] Switched to use Python if syntax and not Ruby's.

Add checking to pause/resume functions in printcore.py

Confirmation on exit

Make sure to set target temps to 0 before exiting.

To ensure safety, we make sure we set target temps to 0 before we exit
pronsole.

Presently, one might accidentally terminate a print with a CTR-D while
printing. Now we ask the user to confirm exiting if there is a print
going when they try to exit.

Protect the user from typos in setting the extruder temp by confirming
them with the user if they exceed 250.

Conflicts:
	gcoder.py

Support M82/M83 for relative E

Show connection error to the user

Clean up echoed firmware lines, connect info.

Presently, when you connect to a printer using pronsole, one sees
something like:

uninitialized>start
uninitialized>Printer is now online
uninitialized>echo: External Reset
printer>Marlin 1.0.0 RC2
...

With a few carriage returns and some string hackery, we clean up
the output so that one sees:

No port specified - connecting to /dev/ttyACM0 at 115200bps
start
Printer is now online
External Reset
Marlin 1.0.0 RC2

Much cleaner!

Clean up the monitor command.

The monitor command now:

* Has more pythonic code
* Limits precision of progress elements (12.3% instead of 12.347812...%)
* Uses a carriage return to have print progress replace the previous
  progress line.

For example:

Monitoring printer, use ^C to interrupt.
Updating values every 5.000000 seconds.
Print progress: 0.3%

Previously, the line "Print progress: 0.3%" was "Print progress: 0.2%",
etc.

Fancy configurable prompts!

* Prompts are now generated based off of string templates,
  for example: "%(bold)sT:%(extruder_temp_fancy)s %(progress_fancy)s
  >%(normal)s "

* We have a dictionary of prompt string templates for different
  situations.

* We have bold support for the prompt.

* We have extruder temperature support for the prompt.

* We have progress support for the prompt.