Support FilteredAdapter interface.

[shy1st]

Fix: #24

[coveralls]

Pull Request Test Coverage Report for Build 90

• 187 of 216 (86.57%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.6%) to 85.227%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/org/casbin/adapter/JDBCAdapter.java	30	35	85.71%
src/main/java/org/casbin/adapter/JDBCBaseAdapter.java	157	181	86.74%

Totals
Change from base Build 84:	2.6%
Covered Lines:	225
Relevant Lines:	264

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💛 - Coveralls

[hsluoyz]

@shy1st see how I wrote the code for jCasbin: https://github.com/casbin/jcasbin/tree/master/src/main/java/org/casbin/jcasbin/main

We have defined 6 Enforcer classes to separate the code into multiple files. But they provide the single Enforcer class to the end user.

[hsluoyz]

@shreyasGit plz review.

[hsluoyz]

lgtm

[hsluoyz]

@shy1st PR merged, plz make a new release on Maven & GitHub Releases.