Add option to skip file renaming

- Added new option g:pad_rename_files to enable disable
  file renaming (default is 1)
- If g:pad_rename_files is set as 0 then
  pad_local.update() will not rename the file
Use first_line as file name for OpenPad

pad#OpenPad() can take a first_line argument to be placed
in the file, however the initial file name is still
set to the timestamp. This commit changes the initial file
name to match the given line

- Replaced timestamp() with a call to PadInfo(...).id in OpenPad
- Removed  (now unused) timestamp import
- Added a docstring to clarify the use of PadInfo with a list of
  strings
Merge pull request #44 from equalsraf/tb-no-renaming

Don't rename files and use first_line as filename for OpenPad