Fixing ArtifactID

I was trying to compile from Maven using 'com.loopj.android:async-http-client:1.4.3'
which doesn't work. It should be:
'com.loopj.android:android-async-http:1.4.3'
Merge pull request #344 from dvdrlee/patch-1

ArtifactID should be 'android-async-http' in readme