Added a progress bar and fixed a bug with regards to error reporting

[joshuahonig]

I've made two changes, and tried to adopt the code style of the repository as best I could.

The changes to the README, imapbox.py, and all lines in mailboxresource.py except for 98 involve adding a progress bar to indicate that the program isn't frozen and to give progress and estimated time remaining. I've gone with instructing the user to install progress, as I was unsure of copying the progress files to this repository due to its license, but this would be easy to change if so desired.

The modification to line 98 in mailboxresource.py is a bug fix; e would technically have the strerror property, triggering the if statement. I added a condition to check that the strerror isn't None.

I was originally only going to create a pull request for the bug fix, and thought I might as well add a progress bar while I was mucking around in the code :)