Fix deprecation warnings due to invalid escape sequences.

[tirkarthi]

Fixes #71

[techtonik]

I could confirm the deprecation warnings on Python 3:

$ python3 -Walways -m py_compile patch.py 
patch.py:340: DeprecationWarning: invalid escape sequence \d
  re_hunk_start = re.compile(b"^@@ -(\d+)(,(\d+))? \+(\d+)(,(\d+))? @@")
patch.py:519: DeprecationWarning: invalid escape sequence \+
  re_filename = b"^\+\+\+ ([^\t]+)"
patch.py:545: DeprecationWarning: invalid escape sequence \d
  match = re.match(b"^@@ -(\d+)(,(\d+))? \+(\d+)(,(\d+))? @@(.*)", line)

Thanks! =)