Unicode support for param.String

[philippjfr]

This commit adds unicode support for param.String, which has been requested on multiple occasions, see #73 and holoviz/holoviews#29. This is the simplest possible fix that will get param to support Unicode strings in Python 2.

To handle unicode correctly consistently we should consider importing: from future import unicode_literals everywhere, but I'm not sure what the implications of that change would be.

The fully verbose version providing compatibility for all string, bytes and unicode types is the following but that's probably overkill:

try:
    unicode = unicode
except NameError:
    # 'unicode' is undefined, must be Python 3
    str = str
    unicode = str
    bytes = bytes
    basestring = (str,bytes)
else:
    # 'unicode' exists, must be Python 2
    str = str
    unicode = unicode
    bytes = str
    basestring = basestring

[coveralls]

Coverage increased (+0.04%) to 69.77% when pulling 6f04d3f on philippjfr:master into 74e06f2 on ioam:master.

[coveralls]

Coverage increased (+0.04%) to 69.77% when pulling cff232b on philippjfr:master into 74e06f2 on ioam:master.

[coveralls]

Coverage increased (+0.04%) to 69.77% when pulling cff232b on philippjfr:master into 74e06f2 on ioam:master.

[jbednar]

Seems ok to me.

[jlstevens]

I think this is an important thing to support and there is very little code so I will merge it now. Obviously, we can continue to improve it (if necessary) on master.