Add more assertions in order to verify correct behavior of ChainPad

[cjdelisle]

Many assertions were created while hunting for #6 and some of them are good for future verification. Here is a git branch with many good (and bad) assertions, the good ones should be merged.
https://github.com/xwiki-contrib/chainpad/compare/lots-of-assertions?expand=1

[cjdelisle]

0d4eccd