refactor FileResource.inputStream to be easier to use

for any client code which needs to convert a uri-like path to an inputstream
the way to go is ```JRubyFile.create(runtime, path).inputStream()```

the motivation for this PR comes from jruby-openssl-0.9.6.dev:

from the user there comes a String with path to a PEM resource. as it turns out this path can be any uri-like path (like from rubygems: https://github.com/jruby/jruby/blob/master/lib/ruby/stdlib/rubygems/request.rb#L42).

the following boilerplate code to convert a uri-like path into an input-stream:
https://github.com/jruby/jruby-openssl/commit/aa51d9ba1d55dfad31943fadc95c8889ebbe456d#diff-67972a6a9364f41a90295ceabf0e9180R298
is a fix for jruby-1.7.16.1

this PR cleans up the API for such "conversion" and to release jruby-openssl with this clean API it needs a released jruby version with this PR merged.

I'd keep rationale for closing (implementations allocating native memory) in the new comment.