Draft: Inefficient Appendable Append fix for spotbugs/spotbugs#704

[manishaghaisas]

Fixes spotbugs/spotbugs#704
New performance rule for Appendable appending CharSequence

Brief description of the files submitted:-
InefficientAppendableAppend.java new detector to examine following two signatures of Appendable and report if the CharSequence.toString() is passed as first argument.

append(CharSequence csq)
append(CharSequence csq, int start, int end)

IAA_Sample.java contains samples.
mesages.xml and findbugs.xml - Changes are still WIP.

Opening a draft pull request in order to discuss the following -

1. This detector has some overlap with the InefficientStringBuffering detector. There is duplicate reporting for the following pattern.

StringBuffer sb = new StringBuffer("foo");
 sb.append(sb.toString());

2. The InefficientStringBuffering detector fails to report an issue for the other method signature append(CharSequence csq, int start, int end)
   Would like to discuss the best approach to address both issues.

[manishaghaisas]

@ThrawnCA @mebigfatguy Any of you free to discuss how best to tackle issue of duplicate reporting by InefficientStringBuffering and new detector InefficientAppendableAppend for the following pattern

StringBuffer sb = new StringBuffer("foo");
sb.append(sb.toString());