Improved string support for Python 2.x

[Xion]

I found that support for string contracts in Python 2.x is lacking because the string/str keyword matches only str type. For applications using Unicode strings, that's not really viable.

So I made some improvements in this regard:

• I added unicode keyword that explicitly matches unicode strings (only in 2.x)
• I made str keyword to match str type only (so ANSI strings in 2.x and all/Unicode strings in 3.x)
• I changed string to match both str and unicode in 2.x and just str in 3.x

I'm aware that the last change modifies the original meaning of string keyword for Python 2.x so that it's more lenient. Existing contract specs using string will still allow what they used to allow, of course, but they will now also accept unicode strings in addition to str ones. So it doesn't break existing usage while (I think) making the keyword more intuitive for Python 2.x developers.

Also, the current meaning of string is not really documented anywhere (I had to find it in source, personally) and thus I suppose changing it slightly will not harm any existing usage.

[AndreaCensi]

Hi Xion, great job!

I'll test it and then release a new version in a couple of days.

[AndreaCensi]

Just released version 1.4.0 including this patch.