Add checkstyle
Update license plugin
Check JavaDoc with Checkstyle for all annotations
Move Checkstyle execution to a separate profile for Travis

It can be really annoying for contributors for always fixing the code
due to broken build by Checkstyle even before starting implementing a
feature. That's why the Checkstyle execution is moved to a separate
profile what will be used by Travis CI. Contributors are still
encouraged to run Checkstyle manually before pushing code, but Travis
will reject malformatted code in the PRs anyway.
Adding @return to all method JavaDoc

JDK8 JavaDoc needs @return on all non-void methods. Unfortunetaly
Checkstyle does not know about this
(http://sourceforge.net/p/checkstyle/bugs/543/), so we have to disable
the JavaDocMethod check for now.
Consider numbers in CaseHelper

CaseHelper.camelCaseToSnakeCase completely ignored numbers before this
commit.
Skip processing annotation when holder is null
Add missing licence headers
Merge pull request #1282 from WonderCsabo/1268_considerNumberInCaseHelper

Consider numbers in CaseHelper
Fix androidannotations.log in apt folder
Merge pull request #1289 from yDelouis/1110_fixFileAppender

Fix androidannotations.log in apt folder
Improve fix for log file generated
Merge pull request #1293 from yDelouis/1110_fixFileAppender

Improve fix for log file generated
Merge pull request #1198 from WonderCsabo/checkstyle

Conflicts:
	AndroidAnnotations/androidannotations/src/main/java/org/androidannotations/helper/CaseHelper.java
Remove checkstyle XREF warning
Merge pull request #1283 from WonderCsabo/1207_skipProcessingWhenHolderIsNull

Skip processing annotation when holder is null
Add android-2.3.1.jar to eclipse-dependencies
Update Android dependency of the aa-api project to version 2.3.1
Remove pre Android 2.0 onBackPressed handling

Since we are now targeting 2.3.1 and up.