Encoding attribute in XML changeset is ignored

Current status:
At this time encoding value in XML change set file is being ignored. Encoding is determined by **not mandatory** BOM (using UtfBomAwareReader). If there is no BOM, system encoding is used. 
This lead to undeterministic behaviour - same changeset will have different checksum - based on the system default encoding. Some users already described it in this issue: 
https://liquibase.jira.com/browse/CORE-1503?focusedCommentId=18291&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-18291

My PR:
I replaced UtfBomAwareReader with UtfBomStripperInputStream to avoid these problems:
https://issues.jasig.org/browse/SSP-894
Now the SAX parser uses encoding from  XML header.