Fix Windows relative path logic. Fixes #8

[twolfson]

As discovered in #8, we have path issues on Windows. The source of the issue is the number-based slicing is running into issues with escaped characters on Windows (e.g. F:\\). To resolve this, we are moving away from numbers and onto string replacement.

In this PR:

• Replaced number based slicing with string replacement
  • Store rel_path_start as the string we were previously computing the length of
  • Use os.path.relpath to replace relative path base

Tested in Sublime Text 3 on Windows and Sublime Text 2 on Linux

/cc @noklesta

[noklesta]

Great, thanks! I've tested this on ST2 and 3 on OSX and Windows 7, and it seems to work fine :)

[twolfson]

Np, thanks for the quick merge =)