Add Android.mk build file and pre-generated config.h to build natively on Android as a static library.

Tested: general event support, socket support, pthread support
Not tested: openssl

To use in a libevent-powered application, add this to the app's Android.mk:

LOCAL_STATIC_LIBRARIES:= libevent2

Based on the latest stable version 2.0.20.
Adding Readme file for Android information.
adding TAILQ_END since it is not defined in Android; adding commented-out BUILD_SHARED_LIBRARY support
Merge branch 'patches-2.0' of github.com:ventureresearch/libevent into patches-2.0
Android.mk: remove bufferevent_openssl.c

OpenSSL should work on Android, but, according to the documentation by
@ventureresearch on LibEvent, it is not supported yet.
Revert "adding TAILQ_END since it is not defined in Android; adding commented-out BUILD_SHARED_LIBRARY support"

This reverts commit 408fa9c38e2f56cfa47169190cd29de79359f673. I have tried to
build without this commit and it does not create any problem, so I deduce it is
no more necessary.