Remove redundant code from disable_logging

This also fixes a minor performance issue because the while loop was
only explicitly exited if logging was not already disabled even if the
buffer had been already found.
Restore default log level after OTR conversation

Fixes #56
Use bash, as `let` is not a POSIX construct

Also, one less line of source code :)
Merge pull request #72 from tribut/logging-reset-default

Reset logging to default if no explicit value was set
Fix indentation

This makes it clearer where the if statement begins
Move print_buffer functionality to global scope
Print "/otr log" error messages to correct buffer
Use user-configurable prefix for output

Fixes #51
Colorize messages according to type
Rework feedback on unencrypted message received

- Use our own prefix to make it clear we modified the message
- Only color the "unencrypted message received" part, leave the original
  message's default color
Merge branch 'colorize-output'

* colorize-output:
  Rework feedback on unencrypted message received
  Colorize messages according to type
  Use user-configurable prefix for output
  Print "/otr log" error messages to correct buffer
  Move print_buffer functionality to global scope
  Fix indentation