ENH: StringMethods supports is_xxx methods
Derived from #9111.

Add following methods to be compat with standard `str`.
- StringMethods.isalnum
- StringMethods.isalpha
- StringMethods.isdigit
- StringMethods.isspace
- StringMethods.islower
- StringMethods.isupper
- StringMethods.istitle

since these functions are quite similar. can you change impl to use a function generator to create these? (with the appropriate doc strings)

It may also be possible to include some of the existing functions in a similar generator.
 Right, how about this?
 looks good. can you add the methods here as well?

http://pandas.pydata.org/pandas-docs/stable/text.html#method-summary (maybe we want to auto-generate this as well from the StringMethods?)
 @jreback Sure. Did it.
 this needs to go after #9322 (and might need a rebase after that) as the docs are changing a bit)
 OK, rebased.
 thanks @sinhrks 
