Add support for Python 3

[tdruez]

File "lib/python3.5/site-packages/license_expression.py", line 324, in build
    if isinstance(expression, basestring) and expression.strip():
NameError: name 'basestring' is not defined

Could be solved with the following:

try:
    unicode = unicode
except NameError:
    str = str
    unicode = str
    bytes = bytes
    basestring = (str, bytes)
else:
    str = str
    unicode = unicode
    bytes = str
    basestring = basestring

[pombredanne]

Fixed with #3 . Next is to add a test matrix for Travis to test on Python3 too

[pombredanne]

Fixed in master